Should I fly if conditions are VFR but the flight briefer says "VFR Flight Not Recommended"? I contacted a flight briefer this morning in the San Francisco Bay Area in California. At the time, ceilings at my airport (Palo Alto) had just hit 1000 ft, so the field was (barely) VFR. It is a very common pattern in this area for the ceiling to burn off, become broken, scattered, and then disappear.

The briefer told me there was an Airmet Sierra for IFR conditions and mountain obscuration in force, and that VFR flight was not recommended. When I talked with an instructor at the club, we talked about the specific location of the clouds and decided the whole area would soon be VFR.

Question: should I fly if conditions are clearly VFR but the flight briefer is telling me “VFR flight not recommended”? How should I think through the risk assessment here?

"VFR Flight Not Recommended" is a judgement call made by the flight briefer. It means, as you noted from the AIM, that the briefer doesn't think flying around VFR is a smart idea based on what they're seeing.

The usual reasons for a briefer using this phrase are:

The weather is currently below VFR minimums somewhere along your intended route.
The weather is currently "Marginal VFR" somewhere along your intended route.
The weather is forecast to be marginal VFR or below somewhere along your intended route.
Some other adverse condition (like mountain obscuration) exists along your route or in the area you intend to fly.

The briefer's recommendation is just that: a recommendation -- it is not binding. How you evaluate the flight to make a Go/No-Go decision when the briefer provides this recommendation is your choice as pilot in command. You will need to consider why the briefer is telling you VFR flight is not recommended, and apply your own knowledge and judgment.

A couple of examples:

If the briefer is telling you this because Airmet Sierra is active for mountain obscuration and you intend to fly through some mountain passes prudence suggests that proceeding VFR into mountains that may be obscured by clouds would be a Bad Idea.
If the briefer is telling you this because the weather is currently marginal VFR, but it's forecast to improve and you only plan to shoot a few touch-and-go landings at your home airport you might proceed with the flight anyway.

For evaluating the huge swath of conditions in between those two extremes consider your familiarity with the area (typical weather patterns, topography, obstacles, etc.) and the conditions of flight (especially day-vs-night). Thomas P. Turner's site has a nice set of guidelines which I happen to broadly agree with, but each flight needs to be carefully evaluated to determine if you believe you can complete it safely under the conditions reported to you by the briefer.

The safest approach is obviously to cancel the flight (or complete it under instrument flight rules) if you have any doubt.

A 1000 feet ceiling means you can only climb to 500 feet AGL to stay VFR.  Maintain that altitude, which on the coast is close to 500' MSL, proceeding inland the ground is climbing rapidly towards you. Rapid deceleration upon contact with the ground might be fatal.  VFR not recommended.


Why separate autopilot controls for course and heading? Is there an explanation better yet equally pithy than the beneath, in fewer than 50 words?

Course is the direction in which the aircraft is flying over the ground. Heading is the direction in which the aircraft is pointing.

This quote motivated me to question the quote beneath:

[2.] See it like this... When Tomtom directs you from Amsterdam to Barcelona via Paris, Barcelona is your final 'course', but for now... you're 'heading' towards Paris... (taking in mind that you just left from Amsterdam...)
[3.] Other example: If you're doing an approach for landing at, let's say, LEBB (Bilbao) Airport on RWY 30, your Course is 300 but you can have various 'headings' TOWARDS your final course, Course 300 for RWY 30.

Why would heading and course need be controlled separately? If I'm flying to Paris first, then wouldn't I just input the heading to fly to Paris? Only after reaching Paris would I change the heading for Barcelona. Isn't 'course' redundant?

In the 'other example' above, isn't 'heading' redundant? Wouldn't I just input the course for Rwy 30 which is by definition 300?

Does anything above explain this picture, where the crosswind requires the plane to angle its nose to land?

First, I'll try for the 50 words or less: Course is the line across the ground that you want to go. Heading is where you have to point the aircraft to fly that line.

That's less than 50 words, but I'm not sure it's any better.

Let's look at the obvious first. Let's say, over a short distance, you want to fly a course of 270 degrees true. If you fly the airplane on a heading of 270 degrees, you will fly that course ONLY if there is no wind, which for all practical purposes is never the case. If there was a wind from the north, you would have to maintain a heading greater than 270 degrees to make good a course of 270 degrees.

Actually, you can never say that the course between point A and point B is a single value in degrees unless those two points are very close together. If you draw a line between point A and point B and those two points are not close together, and then measure the course leaving point A (the angle between north and the line as it leaves point A) you will find that it is different from the course as you arrive at point B.

You can, however, tell the autopilot to fly a direct course from point A to point B and it will do it, changing the heading as necessary to account for the wind and fly that course.

So, why does an autopilot need to have a "course" function and a "heading" function. In my experience, the greatest single use of the heading function is when a controller is vectoring you and tells you to fly heading so and so. The second is when a reroute from your existing course is necessary, and you use the heading function to keep the airplane going straight ahead for a bit while you redo the revised course setting you want to use.

If you forget to again use the course function after putting in the revised course, the airplane will continue on the heading it was on while you were updating the course, and you will shortly be off course. That mistake has been responsible for aircraft being hundreds of miles off course.

During the first Gulf War, I heard a Canadian controller inform a USAF aircraft that they were over 300 miles off course when Canadian radar picked them up prior to landfall coming in from Europe.

A captain at the first 747 carrier I flew for got over 50 miles off course and was demoted to First Officer. He had forgotten to switch from the heading function to the course function.

Finally, as an added problem, if you choose to define your course in degrees magnetic, you must also account for the change in magnetic variation as you proceed along that course.

Insofar as item 4 is concerned, you (or the autopilot) is going to change heading as necessary to keep the airplane proceeding along the course as defined by the runway centerline. There's a bit of confusion of terms here in that runway directions are typically referred to as the runway heading rather than the runway course.

LePressentiment,

Its simple.  Its all about the wind (and to a lesser degree magnetic variation).

Everything else you've mentioned is completely irrelevant, air traffic control, waypoints etc..... all completely irrelevant.

You calculate your correction angle and you fly that heading.

If ATC ask you to temporarily fly a new heading for traffic avoidance purposes, then its still up to you to figure out how you're going to rejoin your original course (i.e. the headings you'll need to fly taking into account wind) when they are done with you.


In the US, do ultralight aircraft have tail numbers? Just wondering if ultralight aircraft in the United States have tail numbers like the larger general aviation aircraft have?

Assuming that you mean an ultralight as defined in part 103, there is no requirement to be registered or have a tail number. 14 CFR 103.7(c) says:

  Notwithstanding any other section pertaining to registration and marking of aircraft, ultralight vehicles are not required to be registered or to bear markings of any type

In the USA: "ultralight aircraft" as in having an engine or two? Yes they do.  Pretty sure anything larger than an 8 oz drone needs to be registered to fly in the US airspace. Hang gliders and parachutes are about the only thing not needing a tail number.  I'm sure someone here can find the appropriate FAR paragraphs with the details from www.faa.gov  My internet is awfully slow today.

I may have overstated things above. Found this article at the United States Hang Gliding and Paragliding Association website, which seems to state that "true ultralights" may not need to be registered, and has quite a few reference links at the bottom:

  January 31, 2008 is the deadline for converting overweight or two-place ultralights used in our current fleet of tugs to FAA-registered E-LSAs. After that date, it will not be possible to convert an existing unregistered tug to an E-LSA. An overweight or two-place ultralight tug that is not converted to an E-LSA by January 31, 2008 can continue to be flown until January 31, 2010, but after that date that it cannot be flown at all, as the FAA will consider it to be an unregistered aircraft without an airworthiness certificate. Very light tugs that actually meet the definition of a true ultralight may continue to tow hang gliders under Part 103 rules, without being N-numbered, as long as USHPA's aerotowing exemption #4144 remains in effect.


Is it bad form to call "ready to depart" when you know you won't be cleared yet? My local aerodrome is towered and has parallel hard and grass runways. Simultaneous parallel departures or landings are not normally allowed. On one occasion we had been instructed to "line up and wait" on the grass runway. While I was performing the pre-take-off checks, an arriving aircraft was cleared for touch-and-go on the main runway. I observed it on short final as I completed my checks, and called "ready to depart", even though I knew that we would not be granted take-off clearance until it was well into its climb-out.

My instructor later told me that asking for clearance when you know you can't have it yet is bad form, and that he would have told me not to, if he had seen the traffic (which had been in his blind spot). I can see how it could seem like impatience or poor situational awareness, but it seemed to me like it helps the controller by letting them know they can get me off the runway as soon as they're able. Additionally, I've been taught that in other situations it's good to get a call in as soon as I can and there is clear air (such as downwind and final calls).

I'd like to get some clarification on this. Is it bad form? Why: for one of the reasons I suggested, or because of some other reason? What other situations does this generalise to?

Informing the controller that you are ready to depart and requesting a takeoff clearance aren't the same thing.

It isn't bad form to inform the controller when you're ready for the next phase; if the controller isn't ready yet, you won't receive a takeoff clearance, but the good news is that now the controller is ready to get you going as soon as possible since your intentions have been declared, and she also has some time to think about what will happen next.

It opens up time for the controller to plan and coordinate.

Situation A:  You wait until the landing traffic is clear of the runway before you state that you're ready for departure, and then the tower clears you; you takeoff.

In this situation, the tower had a short time to react to your request and possibly plan other traffic around you.

Situation B:  You state that you're ready for departure, the tower acknowledges and requests for you to hold short for landing traffic. The traffic lands, and then, the tower clears you; you takeoff.

In this situation, the tower could have altered other inbound traffic to provider a bigger safety buffer for you due to the increased time given to asses the current traffic conditions at the airport.

You always notify the tower of your intentions as soon as you are ready, so long as you are not talking over somebody else.

  Portland Tower, Cessna Seven Niner Two Mike Hotel, number one at two
  eight Lima, departing to the north west.

Say who you are, where you are and what you want to do (depart, remain in the pattern, etc).


Why did my flight climb so sharply on takeoff? I was on a Ryanair flight this morning that seemed to perform an incredibly aggressive takeoff from London Stansted on a 737-8.  Takeoff was fairly fast, but the climb was the steepest I ever encountered.

For context, we were taking off from runway 22, with wind 34 kt from the South/Southwest gusting 51 kt

Your flight took off during a storm. During a storm, the wind speed close to the surface of the earth is much lower than the wind speed a bit higher up. This variation of wind speed over a short vertical distance is called wind shear. The aircraft is taking off into the wind, so during the initial climb the headwind increases. Increasing headwind during climb does miracles to the climb rate.

Suppose you want to climb from sea level to 2000 feet while increasing the true airspeed from 140 to 160 knots (70 m/s to 80 m/s).  In steady wind conditions that means the aircraft needs to accelerate 20 knots during that climb, which consumes part of the energy available from the engines.

But if there is a windshear and the headwind is increasing by 30 knots during that climb (not unreasonable in stormy conditions), the ground speed will reduce by 10 knots. Instead of needing to add kinetic energy, the aircraft needs to reduce it by converting it into potential energy (altitude). This means more energy is available for climbing and thus the climb rate is higher.

The wind reports from the airport at the time of take-off showed a wind of 29 knots (39 knots gusts), almost straight down the runway.

If you look at the wind profile this morning (I could not access yesterday's data anymore) you see that the wind increases from 20 kts at the surface to 51 knots at 3000 ft. That's a lot of free airspeed the aircraft gets during climb!

 source: screenshot from windy.com

 source: screenshot from windy.com

In addition, the effect of a stormy headwind is that the ground speed is relatively low. This makes the climb much steeper than in the no-wind condition.

Looking a bit closer at the raw data from FR24, the aircraft took off at 131 knots groundspeed (the first airborne report is 25 ft above the runway). 6 seconds after the first airborne ADS-B report, the altitude had increased by 525 ft. Even if we conservatively assume it took 7 seconds to climb to 500 ft (round-off errors), it requires a vertical acceleration over 1.6 g to achieve that. That must have felt very aggressive!

The first 2000 ft of climb took only 25 seconds, an average of 4800 ft per minute. The ground speed reduced to 119 knots by the time the aircraft had climbed 2800 ft, 37 seconds after take-off.

After that, the aircraft had climbed through most of the boundary layer of the earth and the windshear reduced. The rest of the climb was fairly typical.

It was not only your perception; by all means it was an impressive take-off performance for a B738

It was due to the wind shear. Wind shear creates a vector gradient. If the vector gradient direction is upward in the head you'll get additional vertical force for lifting and its magnitude is larger than the downward force in the tail. That's why it's called wind shear because the wind applies a shear force to a flying body (in this case airplane). That's will happen only when the high-speed wind is heading towards your plane and your plane take off with a rotational velocity (Vr) designed for a steady condition. As force can be modeled as a rate of change of momentum over time it makes sense that you get a sudden steep climbing pattern because the momentum of your plane has been increased.
Wind shear is classified as an atmospheric disturbance. Therefore, it is correlated directly to the flight qualities. You can check the table in Flight Dynamic Principle Chapter 14 to see the relationship. As long as you can maintain control there will be no problem. For the passenger's complaint, they have the right to complaints as a part of the whole system. A complaint is a gift to improve the overall system the airlines, the pilot skills, and the aircraft manufacturers.
The only thing to be concerned from your case is communications. You should get a weather report before take off. Someday, this communication issue can be reduced by visualizing the wind. I guess everybody would be happy if we are able to see the wind shear. Some airports have the technology to visualize wind shear. It's just the implementation of the Tyndall effect (at least for a while). Maybe in the future, everyone will able to see the wind shear using 3D light technology. The plasma emission technology is the one and only 3D light that we have today and it's dangerous. The plasma emission phenomenon near the cloud full of static electric charge will transform your Airplane to a giant spark plug. For now, just trust the pilot they're trained well. If you're a pilot just trust the aircraft manufacturers they've been work so hard to keep everybody safe and of course, communicate well with the ATC & ground staff.


Can helicopters create contrails? Can some or all helicopters have contrails? If only some, why only those? If not, why can they not? Pictures would be really helpful. I do know that turboprops and piston props can leave contrails, but I haven't been able to find anything about contrails on helicopters.

If the temperature and conditions are right, yes.

Any aircraft which has a heat engine that burns hydrocarbon fuel, which is pretty much any reciprocating or gas turbine engine used in aviation, will emit carbon dioxide and water vapor as byproducts of combustion.  It’s just a byproduct of the chemical reaction to produce heat energy in the engine. And when the temperatures and humidity in the ambient air are low enough and high enough respectively, the water vapor in the exhaust stream will freeze into ice crystals that remain suspended in the atmosphere for a sustained period of time.

You typically won’t see this at your average airport or from the average helicopter flying over you as the temperatures simply are not low enough at the altitude the aircraft operates at for contrails to properly form. That being said, see picture below.

Image source: https://koreaaero.tistory.com/40 (in Korean)

In a word: NO.

All existing helicopters have their engine exhausts somewhere beneath the main rotor, and that means that the exhaust gases are hugely diluted and dispersed by the enormous downwash from the main rotor. It simply is not possible for a contrail to form under such dilution and turbulence.


How do aircraft keep fuel from freezing while in flight? On aircraft which fly above 30,000 feet, the temperature can often can get to temperatures below Jet-A's freezing point.

For example: at 36,000 feet the standard outside air temperature is -56.5°C (-69.7°F) and is well below Jet-A's freezing point of -40°C/F.

What measures are put in place on aircraft to mitigate the risk of the fuel in the tanks freezing and preventing the engines from operating?

Much like water, exposing fuel to below freezing temperatures does not instantly make it freeze.  It takes some amount of time, and as long as the fuel is not exposed to below freezing temperatures long enough to make it freeze, it won't be a problem.

How much time?  That depends on a number of factors like:

Type of fuel

Jet A (common in the US) has a freezing point of -40 C
Jet A1 (common outside the US) has a freezing point of -47 C
Jet B (common for some military aircraft and very cold airports) has a freezing point of -60 C

Initial fuel temperature

If you fuel in the middle east during the middle of summer, the fuel will be a lot warmer than when you fuel in Siberia in the middle of winter.  Warmer fuel will take longer to freeze.

Outside air temperature

Much like trying to freeze water to form an ice cube, the colder the air around it, the faster that it will freeze.

Aircraft speed

The faster that the airplane goes, the more friction there is as the air flows over the wing.  This heats up the air close to the wing and makes it take longer for the fuel to freeze (see previous point).  The temperature at the wing is known as TAT:
For example, at an actual temperature of -56.5 C (SAT) and Mach 0.72 the TAT is -34 C, but at Mach 0.80 the TAT is -29 C.
For more information about SAT -vs- TAT, see my answer about Air Data Computers.

Quantity of fuel

Much like a small pond will freeze before a large lake, in order to get fuel to freeze you need to lower the temperature of all of it.
The more that there is, the longer it will take.

Aircraft design

Some aircraft run hydraulic lines close to the fuel so that the heat will help slow down the freezing.
Some aircraft constantly pump fuel to keep it moving and mixing if it tends to freeze in one part of the tank before the rest
Some aircraft use fuel to cool the engine oil using a heat exchanger, and return the warmed fuel to the tanks
Many other design factors

In general, fuel will not freeze in the typical amount of time that an aircraft is aloft.  If particular routes pose a problem, there are several options that the operator and pilot have:

Use a fuel with a lower freezing point
Fly at a lower altitude where it is not as cold
Choose a route with warmer temperatures
Increase the speed of the airplane
Carry extra fuel
Transfer fuel in a way to keep fuel warmer.  Typically this is done by transferring fuel from the relatively warm fuselage tanks to the wing tanks which are exposed to more of the cold air.

Since the airplane is in motion, so is the fuel. That principle alone keeps it from freezing. As the fuel agitates, the molecules are prevented from freezing because of motion. This only works to a certain degree.


Would a 19,000 ft descent in 6 minutes feel noticably different to passengers? In July 2008 a Qantas 747 suffered decompression at 29,000 ft and the pilots then descended to 10,000 ft in the space of six minutes . More details here : wikipedia.org/.../Qantas_Flight, 30 .

I'm curious to know if you're in a large aircraft descending at that rate would there be a strong sensation of descent for the passengers? Would the plane be angled downwards in a way that wouldn't normally happen or some other sensation not normally experienced when travelling on large civil aircraft?

I appreciate that in this case the masks were down and there might have been noise/wind associated with the actual incident, so clearly it did feel strange to these passengers, but if the descent was done as a test at that rate would it feel strange?

From the physical side, you can not measure / feel if you are at a constant altitude or if you are ascending / descending with constant speed. What you can feel is the change of speed in any direction, and acceleration acting in an unexpected direction. This causes the usual sensations:

Accelerating / braking on the runway: You are pressed into the seat / belt.
Weird feeling right when the aircraft becomes airborne: The aircraft accelerates upwards, which presses you into the seat with more than 1g. This higher g-force feels strange.
While climbing, the aircraft still accelerates, but the more strange feeling comes from the fact that the force doesn't point down to the floor of the aircraft, but more backwards. Your eyes see that you are sitting upright in the aircraft, but it feels more like lying.
At some point, usually when the flaps are retracted, the climb rate decreases suddenly and it feels as if the aircraft falls. This is because the aircraft accelerated downwards - tough it still climbs, but at lower rate.
Usually, you don't feel much during a coordinated turn. Maybe you feel a little heavier, but when you look out of the window and don't see horizon, this again gives a strange feeling.

Of course, the passengers of that flight did have several sensations. right after the incident, the aircraft for sure has done some unusual movements, plus what the pilots did.

The reference #7 of the Wikipedia article contains this diagram on page 36:

Directly after the incident, the aircraft climbed a little and the speed brakes were deployed (50%) to reduce speed. This was for sure noticeable by the passengers. And I guess the aircraft started to descend faster than usual, so the sensation of "becoming lighter" will have been much stronger/longer than usual.
After this, the aircraft descended with a quite constant rate for several minutes. This doesn't cause the feeling of being lighter. Maybe the attitude was unusual which means gravity doesn't push you vertically into the seat.

Later, when the aircraft reached 10,000ft, it accelerated upwards to stop the descent. At this moment, the passengers felt heavier.

(If you have a closer look, the descending part isn't a perfect straight line. This wasn't a smooth ride, but as said, a constant descent rate doesn't cause a sensation)

Of course, this is just about the sensation of acceleration so far. The passengers for sure have noticed the drop in cabin pressure, drop in temperature, more noise and so on.

Yes, it would feel like an elevator descending very quickly. Elevators in small buildings generally descend at around 500ft/min. As an aviator you're taught that this is a comfortable descent rate for almost everyone, and it's what most General Aviation (GA) pilots shoot for. Elevators in most skyscrapers operate up to 2000ft/min. At 2000ft/min a human body reaches roughly one-fifth of its free fall velocity, making a person feel 20% lighter. This is a noticeable and sometimes nauseating effect. The other way that people feel descent rates is through pressure changes, often making ears pop and hurting young children. This isn't as much of a factor for commercial flights as their pressure changes only slightly above 10,000ft MSL.

I mentioned that most GA aircraft ascend/descend around 500/ft min for comfort. Most turbine or commercial aircraft will ascend/descend on instrument clearances that call for rates of 1000ft/min - 1500ft/min.

The pilots in the Qantas case did a great job. They achieved the perfect glide slope for that aircraft and conditions. Gliding that plane for 20 minutes would have resulted in a splashdown for most pilots.

Anectdotally, I am a pilot and have descended at 3000+ ft/min and can tell you it is VERY disconcerting if you're not conditioned for it.


Do horizontal stabiliser force direction change with CG variation for a given aircraft? I am wondering about the direction in which horizontal stabilizer provide force. I know that it can provide either upforce or downforce. But I am wondering whether is it determined for a particular airplane, I mean it is established for example that B737 tail will provide only a downforce and CG limits are set up in a way that when correctly loaded the tailplane will provide every single time a downforce or it depends from the CG location that tailplane will have to provide either a downforce or an upforce?

The ideal cruise condition, which minimises induced drag, is for the tail to exert zero lift. This avoids any wasteful tip vortices from the tail.

However for most planes the CG shifts during flight, if only because fuel is being used up. Also, trim conditions change with speed, rate of climb, altitude and a host of other variables during flight. The tail spends most of its time a little away from the ideal zero-lift condition, and it can go either way.

The "lifting tail" was much discussed in the early years of aviation. It has a destabilising effect which limits the stable CG range and every plane has a maximum safe level of lift from the tail while remaining stable. Nevertheless, away from that limit it contributes lift which helps to offload the wing and allows it to be that bit smaller.
According to Berriman; Aviation, Methuen, 1913.

"On some machines the tail planes are cambered like the wings and help to support the weight in flight. The angle of incidence of the tail is less than that of the wings, however, and the principle [of weathercock stability] apples equally to such cases." -- Page 69.

Berriman gives some supporting analysis in terms of CG and CP to explain what he summarises as weathercock stability. The subtleties of the lifting tail would be further elaborated in subsequent years.

The bigger the tail, the greater the CG range can be accommodated, and also the greater the temptation to offload the wing a little. The limit is the tandem-wing aircraft, where for stability the fore wing is loaded more heavily than the aft, but the aft still contributes substantially to the lift under all flight conditions. A tandem-wing modification of the Westland Lysander flew experimentally during WWII and test pilot Harald Penrose confirmed that its handling was more stable than the conventional version and had a greater CG range.

So ultimately, every plane has its own practical tail loading, which may be normally up, normally down, or variable around the zero point.

There is a myth that for a plane to be stable the tail must always exert a downforce. This seems to arise among students for two reasons. One is that, when the AoA increases, dynamically it must exert a larger relative change in downward moment about the aerodynamic center than the wing's upward moment. Diagrams showing this effect always have a large down-arrow at the tail and it is easy for the student to forget that this is a relative change not an absolute value. The other is that the wing works hardest at takeoff, when fully laden and at its slowest flying speed; when the tail pushes sharply down to rotate the nose up, the wing has to compensate for that as well, so this is the design condition for sizing the wing and everybody has to learn it. Put that first diagram alongside and you have a myth in the making. If you failed to understand any of this paragraph, then you too may fall for it!

In stabilized steady state flight, if the tail is providing lift UP, it means it is opposing a net pitch up moment (it's lifting to keep the nose from rising).  To have a net pitch up moment, the CG must be aft of the "mean center of lift" or Neutral Point, in order to create a nose up torque resultant from mass acting about NP.  This is a dynamically unstable condition and only FBW fighters can operate this way.

In any normal airplane, the CG must always be forward of the Neutral Point, this distance being called "positive static margin".  A positive static margin means there is always a net nose down pitching moment present and the tail has to provide down force to balance it out.  The only time the tail will lift up is during maneuvering cases where a stick forward control input is sufficient to pitch the airplane over faster than the natural pitching moment can do it.  Then the tail will be lifting, briefly.

If the tail was lifting in any normal steady state case, it would mean that that Static Margin is negative, and that to slow down, the trimable tail's incidence would have to move leading edge up and the elevator move down.  I am not aware of any aircraft in existence in this universe with a conventional tail where the elevator has to move down, or the stab has to move LE up, to slow down.

From Boeing:

  1 Positive and Relaxed Longitudinal Static Stability
  In flight, the wings of a conventional airplane generate a nose down pitching moment. To balance this, a download is required on the tail. Airplanes loaded with an aft forward CG require less download on the tail.

Since download on the tail is negative lift, effectively increasing the weight of the airplane, the location of the CG affects the cruise performance of any airplane. Flying at an aft CG will reduce the download on the tail and improve cruise performance.

When airplanes are operated near the aft CG limit, download on the tail is minimized and angles of attack and drag are reduced. However, moving the CG aft reduces the longitudinal static stability of the airplane, something that all flight crews should be aware of (see figure 1 and "Static Longitudinal Stability and Speed Stability" below).

As airspeed varies from a trimmed condition, the column force required to maintain a new speed (without re-trimming) is a measure of static longitudinal stability. For any conventional airplane, the location of the CG has the strongest influence on static longitudinal stability. For a statically stable airplane the required column force, as speed varies from the trimmed condition, is less at an aft CG than it is at a forward CG. The minimum average gradient allowed by U.S. Federal Aviation Administration FAR Part 25 is one pound for each six knots. As the CG moves aft, it reaches a point where the stick force per knot drops to zero, then reverses. This location is called the neutral point. The difference between the actual CG location and the neutral point is called the static margin. With a CG forward of the neutral point, an airplane has a positive static margin and positive static longitudinal stability. At a CG aft of the neutral point, an airplane has a negative static margin, is statically unstable, and requires some form of augmentation to be flown with an acceptable workload.<


What is the appropriate fuel tank selection to land a 172? On a recent checkout on a C172 I was told to select "fullest tank" as opposed to "both" that I have been doing for years. Given the aircraft is high wing (gravity feed) with no auxiliary fuel pump there should be no chance of sucking air.

Anyone know the correct selection?

You should check your POH but it may just be that the instructor in question was recently flying a lot of Pipers and it was force of [bad] habit.

This POH for the 172S is in agreement with you.

  BEFORE LANDING

  ....

  Fuel Selector Valve -- BOTH.

You should consult the checklist in the POH that is with the airplane in question to be sure.

Before landing, you should inspect the fuel qty indicators for and imbalance.  If one exists, select the fuller tank, if none exists , select BOTH.  You will want to minimize the weight imbalance between the tanks and have the engine help address this by burning off fuel from the fuller tank.  You should also monitor your fuel levels in flight and make aproroate rank switches as needed th keep the fuel levels more or less even if an imbalance comes about with the valve on BOTH.


Is the altimeter setting corrected for temperature? In an ATIS or AWOS/ASOS recording, is the reported altimeter setting corrected for temperature?

I think it must be, because on a hot day, the altimeter should still show correct field elevation. If it wasn’t corrected for temperature, then the reported field elevation could be badly wrong on a really hot or really cold day.

(This and other questions describe pressure and density more generally.)

The reported altimeter setting, QNH, is 'somewhat' corrected for temperature, in that it corrects for temperature error at the aerodrome elevation. This is the main difference between the more-meteorologicaly-inclined QFF which does not contain this temperature correction.

You will still get altimeter errors because of the high/low temperature while being above (or below) the aerodrome  (about 40 ft for each 1000ft for each 10 degrees C), but the actual temperature error at aerodrome level will be 0.

The altimeter is adjusted for air pressure, which is a function of temperature (and altitude). So the answer to your question is yes.

That is why it is bad for an airport to be "hot and high" (think Phoenix in the summer), the high altitude is compounded with nearly 100°F (40C) temps, makes for very thin air.


What is the maximum number of planes a controller can control? In a small and un-busy airport, the same controller can be the ground and tower at the same time. If the sector is un-busy (e.g. late at night), he may also be a sector, ground and tower all at the same time. At busy airports of course, ground and tower are two persons. Sometimes even ground is split to "Ground South" and "Ground North".

So there is a threshold where the communication has to be split into two, either because the frequency would be too busy (only one person can talk on the frequency at any time), or there are so many planes that a controller would start making mistakes.

What is that threshold? (Is it a concrete number, or some other conditions which can be objectively determined?)

There is no fix maximum a controller is allowed to have under control, as the other answer already pointed out. The airspace under control of ATC is divided up into sectors containing the airways to and from airports within that sector or in upper sectors, airways just for en-route traffic. (see also: Why do pilots use airways instead of GPS direct?).

Sectors can be merged during off-peak hours, split back into separate sectors during peak hours and in the case of TMA or approach sectors, the controller owning the sector can receive additional help by splitting the approach sector further and using different controllers for different tasks within that sector: Feeder/Pickup - Director - Departure Controller. The thresholds of when a sector is split is dependent on many factors, like weather, runway in use, outside influences like adjacent sectors having more traffic that usual, special occurances within that sector like parachute jumper drops.

Sectors also vary in size and capacity based on their intended use. While TMA/Approach sectors are rather small and need to accomodate arriving traffic into an airfield, an enroute or upper sector can be significantly larger in size, lateral and vertical. Again, this is individual to each sector, because some factors which hold true for TMA sectors do not apply to enroute upper sectors and vice versa. It also really depends on the controller working the sector and his personal limits, favorite procedures and work practice. A few examples:

A TMA controller using vectors to the approach instead of standard arrival routes or transitions will have more workload to monitor and sequence aircraft.
The minimum radar separation in TMA sectors and upper sectors can be either 3NM or 5NM. While upper sectors are larger in size, they have to maintain a greater separation, so in some cases, an upper sector twice or multiple times the size of a TMA sector will only be able to accomodate the same number of aircraft (ceteris paribus) before reaching saturation.
A controller having to work with special situations such as aircraft in distress or student pilots requiring more attention than experienced pilots, will reduce the controllers personal capacity.

In summary, to answer your question: There is a situation and controller based threshold, at which saturation will be reached. Controllers are trained to anticipate this threshold and make their supervisor aware of the situation beforehand, so that measures can be taken. There is however no fixed numerical maximum, unless specified in the ARTCC/ACC's operating procedures.

There is no limit, really. The question is not whether the controller would "start making mistakes", it is how much attention would the controller give to each plane, which just grows less and less the more aircraft there are.

Even if the tower goes completely dark, the pilots will self-organize and land themselves. The controller is just there to make the process easier and safer.

Sometimes controllers do ignore you when they get busy with something else and what happens is you end up flying really LONG downwinds, so by the time the controller gets around to you, you turn onto a 5-mile final or something like that. If the controller completely blows you off, you can leave the pattern, do a U-turn, come around and re-enter the pattern to try again.

At a big airport with a lot of commercial aircraft, if the controllers started to go AWOL the pilots would probably just stack themselves up on the instrument track, and even start landing themselves if the tower became totally unresponsive. To do this, they just tell the other pilots where they are and their position in the queue ("Lufthansa One Echo Echo downwind for two four left, number three behind the Embraer", etc).


Why does the Zapata Flyboard Air UL consume so much more fuel/hr than the Mosquito Air? They are both ultralight and abide by FAA ultralight reg 14 CFR part 103.  The Mosquito Air is an ultralight 1 person helicopter whereas the Zapata Air UL is a micro jet turbine powered 1 person ultralight hoverboard. They weigh the same and both only carry 5 gallons of fuel as this is dictated by part 103.

The 5 gallons will last 1 hour of cruise flight time on the Mosquito while only 8 minutes on the Zapata.  Granted we are dealing with different technologies here and different fuels (Zapata uses kerosene, Mosquito uses avgas) I was surprised by the fuel burn difference.

The bare physics tells us that equal masses should require the same amount of thrust, thus energy, to hover in place where lift equals weight.  Kerosene has similar energy density to avgas.  And, a jet turbine is supposed to be much more efficient than a 2-stroke, 2 cylinder gasoline engine.

The Mosquito boasts of a 64hp engine "with the highest power to weight ratio on the market today".  The Zapata company seems not so much concerned with the gas guzzling fuel consumption.  The 2 companies seem to have different priorities here, so I'm guessing the fuel efficiency of the Zapata could be improved.  Still, this is a huge difference that is difficult to understand.

It's the difference between jet engines and propellers. It is a lot more efficient to accelerate more air to a low velocity, than it is to accelerate less air to a higher velocity. For fixed wings, the only reason to use jets is if propellers hit their speed limit, when the tips break the speed of sound.

Hovering flight is particularly demanding on fuel consumption. The Mosquito Air burns through its fuel a lot faster than 1 hour when it stays in hover all the time. Cruising in a helicopter makes the fuel consumption  a bit more similar to fixed wing: the rotor blades start to act more like wings and can use airspeed to reduce induced drag. Still less efficient than fixed wing cruise, but more efficient than helicopter hover.

The Zapata Air UL is always in a hover, and has no wings, and is a jet. It never gets the increased efficiency effect of forward flight that a rotor gets. And jets are really very inefficient at low speeds!

I know this post is a little old, but I happened to find this post while trying to find out what turbines Zapata uses.  There seems to be a misperception of how helicopters fly/hover compared to how a turbine engines produce thrust.

Helicopters actually do NOT fly by thrusting/pushing air down, although that is, in part, a result of what is going on.  Helicopters create LIFT via the profile of the rotor blades and the angle of attack which varies according to operator input (at least on the bigger boys).  As I'm not a helicopter person, I don't know if tiny copters like the Mosquito can actually vary angle of attack, which makes the mechanism quite a bit more complicated.

If you look at a copter blade profile, it's generally the same as an airplane wing.  The faster it spins (and therefore moves through the air), the more LIFT is generated, and eventually enough lift is generated to lift/propel the copter.

A turbine engine provides THRUST, and it is the THRUST that lifts/propels things like Zapata's board.


Does cargo heat failure require a diversion? What about if there are live animals in cargo? There was a story in the news recently that an Air Canada 787-8, enroute from Tel Aviv (TLV) to Toronto (YYZ), diverted to Frankfurt (FRA) when the pilots discovered a problem with the cargo heat.  There was a live dog in the cargo hold, and the diversion potentially saved it from freezing to death.

The narrative has been that the pilots diverted specifically in order to save the dog.  But I am wondering whether a cargo heat malfunction might require a diversion, as a matter of standard operating procedure, regardless of whether live animals are on board.  After all, even if there are no live animals, there might be other valuable temperature-sensitive cargo.

In general, according to airline policies or procedures, under what circumstances would cargo heat failure warrant a diversion?

Any major airline would be of interest; if you want a specific one, let's say Air Canada.

Does cargo heat failure require diversion? It depends on the cargo being carried.
For your question about pets, then yes.
Council Regulation (EC) No 1/2005 regulates the protection of animals during transport in the EU, which itself is in accordance to IATA Live Animals Regulations. The guidelines based on that EU regulation say:

Commanders, and loadmasters and aircrew under their authority must ensure that the aircraft and equipment are suitable for carriage of the animals concerned in the manner intended. They must ensure that the animals are loaded, carried and unloaded in a way which will protect their welfare.
In particular they must ensure that the animals are accommodated in accordance with the IATA LAR, and that an appropriate environment of air quality and quantity, temperature and pressure is maintained whilst the animals are on the aircraft.

The ability to ventilate and regulate the temperature of an aircraft belly hold varies considerably. The arrangements vary between individual aircraft depending on the make and type, the compartment, and whether or not ‘standard’ or ‘special’ specifications have been fitted.
It is therefore essential that the aircraft operator, who should know the individual features of each of its aircraft, is satisfied that the belly hold to be used can be sufficiently ventilated and maintained at a satisfactory temperature during all stages of the proposed flight. This assessment should take into account the species, number, weight and size, and volume of the animals to be carried, and the heat and moisture that they will produce. Other cargo being carried at the same time will also need to be taken into account.

Even the Minimum Equipment List of aircraft takes that into consideration. For example, the aircraft can be dispatched with an inoperative cargo heater or ventilation fan, but not if there's an animal in that respective cargo hold.
All that confirms the pilot's action in the story you mention.

Common sense applies. The regs don't require a diversion for a cargo heat fail. But, if you know there are live animals down there, and you know it's -53 outside, then you're going to divert to save them.

As the other poster said, if it's just some expensive something else down there, then no.


Why is "Affirm / Negative" used instead of "Yes / No"? Does anyone know why we use Affirm and Negative in RT, rather than Yes and No? I assume it's to prevent confusion with accents and other languages - but Yes and No sound very different to me!

Why do we say "TANGO" on the radio for the letter "T"? Because "T" could get garbled or misunderstood to be "B", "E", "or "V". "NO" could become "oh?", "YES" could become "us", and so on...the possible interpretations and permutations of short, monosyllabic words are many. Use a big word and it will be hard to confuse with others...

Because it increases the number of syllables being spoken thus increasing the likelihood of correct communication.


Why are some windows in cockpits made openable? Why do aircraft allow sliding / opening windows in cockpits? What purpose does it serve, and why the same can't be done for passenger windows as well?

The opening cockpit windows do serve some purpose, like providing an additional means of evacuation during an emergency.* There have been instances where the crew had used the cockpit windows to escape during hijackings.
There are other purposes like assisting in crew inspection, signaling ground crew, additional ventilation, smoke egress etc (It can also be used for throwing things out, though it is not generally recommended). One main reason (atleast in the past) appears to be to improve the pilot visibility in case the windshield becomes opaque, for example.
The windows are not found in all aircraft (for eg. 787 doesn't have it) and are not very easy to open (note that it opens inwards). From USAToday Ask the Captain:

First there is a locking mechanism, then either a crank which moves the entire window assembly including the frame, or a lever with which to move the window assembly. When the window is closed and locked, it is sealed shut. When open, it does not separate from the frame as the window in your car does.

Actually, FAR 25 Section 773- Pilot Compartment View requires cockpit windows to be openable (or some other means to have clear windshield during precipitation) :

(b) Precipitation conditions. For precipitation conditions, the following apply:
(1) The airplane must have a means to maintain a clear portion of the windshield, during precipitation conditions, sufficient for both pilots to have a sufficiently extensive view along the flight path in normal flight attitudes of the airplane.
(3) The first pilot must have a window that—
(i) Is openable under the conditions prescribed in paragraph (b)(1) of this section when the cabin is not pressurized;
(4) The openable window specified in paragraph (b)(3) of this section need not be provided if it is shown that an area of the transparent surface will remain clear sufficient for at least one pilot to land the airplane safely in the event of—
(i) Any system failure or combination of failures which is not extremely improbable, in accordance with § 25.1309, under the precipitation conditions specified in paragraph (b)(1) of this section.
(ii) An encounter with severe hail, birds, or insects.

In a pressurized cabin, providing windows is not a great idea. (In addition to the increased weight), if anyone manages to open the window, it would be a recipe for disaster (depressurization -> oxygen masks -> land).

*Many aircraft have escape ropes mounted in a compartment above the window on each side of the cockpit as in this A320:

Photo source
And a video of it being used.

and why the same can't be done for passenger windows as well?

Anybody who opens even a bit a potential airliner sliding window near his seat creates in the entire plane the conditions existent on the top of Mount Everest, even worse. Nobody wants to stay in such an environment. In conclusion passenger sliding windows serve no purpose.


Why are traditional knobs not replaced by touchscreens in cockpits? Looking at this picture of an A380 cockpit I see that there are still a lot of traditional switches (meaning manual click on and off or knobs). Being technology so advanced in these days, why is it that traditional switches and knobs are used instead of having a couple of touch screens on which the various controls are placed?

Wouldn't this make it more economic as there would be savings on material and wiring? They could also be less susceptible to failures as traditional switches and knobs still rely on mechanical actions.

Another advantage I could think of is that every pilot could customise positioning of controls and switches based on his/her way of piloting. This could then be reset to a default state every time a new pilot takes over.

Adding a pilot's perspective to what others have said:

Look at the switches and knobs in an airplane up close some time. You'll notice that nearly all of them have different textures,heights,sizes or shapes. That's intentional. Pilots train and train and train and train on checklists, especially the emergency ones. Muscle memory is a big part of quickly and correctly executing an emergency checklist.

As an example, flap levers/switches normally have a cap or top that is flat and parallel to the wings. It's easy to identify solely by touch.

Checklist item: "flaps up"
Action: Hand to flap lever (automatic after doing it a hundred times in training), verify feel of lever, move, look to verify

Without the tactile element, every motion would have to be verified by looking before it's made. With the tactile element, the movement can be made and then verified visually while the hand is moving to the next item. In an emergency, every second counts and the savings from tactile + verify matters.

Beyond emergency procedures, they're simply safer. The plane I fly the most has a touch screen display and several non touch devices. In turbulence, the touch screen is basically useless. But, the important devices all have buttons and knobs. I can be banging my head on the ceiling (literally) and still adjust the auto-pilot or radio. In that kind of turbulence, the touch screen device is completely useless.

The A380's maiden flight was in 2005.

iPhone's release was in 2008.

Planes development cycles are drastically longer than phones, and require very reliable materials.

We just didn't had readily available large capacitive touch screen when engineers started designing the A380's cockpit.


How much do propellers stutter? A photo in EAA's monthly magazine shows one propeller blade as a dozen distinct images rather than the usual smooth motion blur.  One explanation (this answer appears to have been deleted) for this is that a propeller at nominally 2300 rpm actually varies +-200 rpm as each cylinder fires, so when it's slower, it captures more sunlight glint.
Edit: this question is about propellers, not about a photo.  The photo is only a motivation, perhaps an incorrect one, for the question.
Is this plausible?

Piston engines have flywheels.
A 400 rpm change happening 2300 times per minute is a 17% change happening 38 times per second.  Jerry Lee Lewis would say that's a whole lotta shakin' goin' on.
Onboard GoPro videos with rolling shutter artifacts show the blades "bending" from root to tip smoothly, not jaggedly.

We can count more than 15 blade-images in that short arc of revolution of the propeller, so there's no way that each one corresponds to an individual firing of a cylinder.  Perhaps there is a resonant vibration involving a slight twisting of the blade, and the sun angle is such that this produces a strobing effect.
The suggestion of a 300 rpm variation between each individual firing of a piston is completely implausible.
Some sort of camera artifact seems a possibility.

It's nothing more than the usual "time aliasing" of any (digital) photo ever taken of any propellor.
There are 1,000 detailed explanations of this online ...
http://resourcemagonline.com/2015/11/why-your-camera-makes-airplane-propellers-look-so-weird/60421/
(Note that if you look carefully at the photo of the blue plane, you can see the typical "bending" as well - as with every single digital photo, ever taken, of a propellor you get two effects (a) bending and (b) repeats. Exactly as in the photo here. Like every single digital photo, ever taken, of a propellor.)
Please note that the answer over here:  https://photo.stackexchange.com/a/121533 (which the writer states is a "theory") is "unbelievably incorrect".

Here's a photo that perfectly shows the "bending effect" will be either more or less depending on the geometry domain.
https://www.dpreview.com/forums/thread/3692327?page=2

This perfectly shows that the "bending effect" can be either extreme or gentle - gentle seen on the blue plane image.
You can see in a zillion photos that you get repeats of the blade (more than the actual number of blades!)  This is completely commonplace and applies to literally every photo, ever, taken digitally of a prop.
http://sbiii.com/aviatn-7.html

The image just above is literally identical to the blue plane image, except the time domain is a bit different.
(For the record, the idea that the propellor could be revolving in a step-wise fashion is bonkers.)
Every single digital photo even taken of a prop shows
A - multiples
B - the bending effect
the blue plane photo is wholly unremarkable (indeed I instantly googled dozens of images where the "multiples are close together" as in the blue plane image).
BTW I realize the headline question here is "how much does a propellor stutter" but the idea is so silly, and the facts on that have already been noted in the other answers.


In the Lockheed Martin Flight Services system, what are MIFR, MVFR, YFR and ZFR flight rules? When I file a flight plan I can file under either IFR or VFR flight rules. However, the Lockheed Martin Flight Services website has the following options in addition to IFR/VFR:

MVFR
MIFR
YFR
ZFR

1 and 2 are only available for US domestic flight plans; 3 and 4 are only available for ICAO plans. What are all these options and when should I use them?

According to the tooltips on the Lockheed Martin site, the flight rules are:

  MVFR - Military VFR; restricted to military flights
  MIFR - Military IFR; restricted to military flights
  YFR - Initially operated under IFR, followed by one or more changes of flight rules
  ZFR - Initially operated under VFR, followed by one or more changes of flight rules

There's very little information on MVFR and MIFR flight plans that I could find, but the FAA's FSS procedures do include specific actions that are required when receiving a military flight plan (section 6-4-7).

YFR and ZFR are ICAO rules; the FAA calls combined VFR/IFR flight plans composite flight plans (see section 5-1-7 in the AIM) and on a US domestic composite flight plan both VFR and IFR should be checked. Interestingly, the Lockheed Martin site doesn't allow that.

MVFR: Minimum/Marginal Visual Flight Rules
This refers to the general weather conditions pilots can expect at the surface. MVFR criteria means a ceiling between 1,000 and 3,000 feet and/or 3 to 5 miles visibility.1, 2
YFR
Flight initially being IFR then going VFR3
...or...
Flights that will initially will be operated under IFR, followed by one or
more changes of flight rules.4
ZFR
Flight initially being VFR then going IFR3
...or...
Flights that will initially will be operated under VFR, followed by one or
more changes of flight rules.4
MIFR: Multi-engine IFR5

1: NOAA National Weather Service

2: YahooAnswer

3: PilotsOfAmerica

4: ForeFlight

5: Canadian Fight Center


Will an axial compressor work with an ideal fluid? Will an axial compressor, like the one described starting on page 275 of Mechanics and Thermodyanmics (PDF link), work if the flow was assumed to be ideal (i.e. incompressible and inviscid)?

Specifically, what mechanism will slow the flow down in the axial direction? It's my understanding that slowing the flow down is the job of the stators, but I fail to see how they accomplish this in the axial direction.  The author states on page 284 that the axial flow velocity through the compressor is made constant by design but never explains how that design is achieved and I'm curious if that design could still be achieved with an incompressible fluid. -Thanks in advance!

First of all, incompressible flow is subsonic, however subsonic flows are not all incompressible. An ideal gas flow may be "incompressible" until it meets the compressor. Even if the vehicle is travelling at supersonic speed, the flow is reduced to subsonic ("incompressible") flow by the intake system before reaching the compressor. At this point the subsonic flow gets compressed. Presumably this is what is meant here.

The compressor stages each reduce the cross-sectional area, while the fans draw the gas backwards to maintain velocity. Thus the gas, ideal or not, must compress to fit the reduced volume available to it. This compression creates heat which raises the gas temperature, further raising its pressure.

Viscosity is necessary to establish a circulatory flow component around each airfoil-shaped blade (the Kutta condition). It is this circulatory component which creates the velocity differentials needed for the Bernoulli principle to create the dynamic pressure differentials which make the blades do their work. Superfluid helium (close to absolute zero) has no viscosity and has been found to exert neither lift nor drag on an immersed body.


How much does it cost to give an airliner a fresh coat of paint? Just a random thought I had: the weather gives a jet's paint job a battering; meaning they need to be resprayed approximately once every 5-10 years. (See How often is a passenger jet aircraft painted?)

Roughly what might a paint job for a 777 like the one shown in the YouTube video below, cost an airline?

Bonus: I believe afterwards that the weight of the plane has to be recalculated, so as not to affect weight-and-balance. How is this calculation done?

According to WallStreetJournal article, Tom Horton stated in an interview that :

  A 777 paint job can cost  \$100,000 to  \$200,000, depending on the number of colors involved, and a smaller Airbus A320 can cost $50,000 or more.

I don't know where you come up with these numbers. I'm a 20,000 corporate pilot and also an aircraft broker dealing in corporate aircraft sales. A Challenger 605 or 604 which is a 10 to 12 passage plane cost from \$120,000 up to \$180,000 for the better shops. I have had several painted over the years with recent quotes in the last 60 days in July 2019 and the lowest price was \$120,000. A 777 would take 10 times as much paint alone.


Private Pilot Compensation I am a new private pilot. My parents want me to take them flying. I do work and have a bit of my own money, but compared to the cost of flying it is not much. My parents paid for most of my training. I am still in high school, so I live with them and they pay for most of my existence. Obviously I cannot be compensated for flying them, but I wonder if there is any legal way for this to happen. The same question can be extended to when I take friends flying. Would it be as simple as me using a card that draws from one of their accounts? Me paying but with their money. Any knowledge/advice is appreciated, I would rather not break FAA regulation on my first flight as a private pilot.

The short answer is that in practice the FAA doesn’t care about “compensation” between family and bona fide friends.
The long answer is that the FAA’s mission is to protect the general public from pilots. (If they also happen to protect pilots, that is a side effect at most.) Your parents and bona fide friends are not the “general public”.
As a practical matter, the FAA could never prove beyond a reasonable doubt in court that any particular dollar that your parents or bona fide friends gave you was for flying vs a gift or for something unrelated, especially when it is likely they would be uncooperative with any investigation or charges against you. If they give you money, and you happen to use some of it to take them flying to a place of your choice, so be it. It’s just not worth the FAA’s time to look too closely at that, so they consciously turn a blind eye.
Where you will get into trouble is flying strangers or acquaintances to a place of their choice. The exact path the money takes is irrelevant; if you accept compensation from the “general public”, then you are operating an illegal charter, and the FAA will hit you like a ton of bricks for that.

I believe you are able to split the maintenance fees with them. Then, as you know, once you obtain a commercial pilots certificate, you can make money. However, until then, you can‘t even have your passengers pay the full maintenance fee/flying fee. Also congrats on your Private Pilot Certificate. Have fun flying!


What would happen if I bank a helicopter then pull hard on the cyclic? Prompted by Simon's response to my question here, I will ask another question about reality.

If I:

Start with a AH-6 helicopter in a hover
Pitch down slightly to accelerate, then slowly increase collective to gain speed until max collective is applied and the airspeed is near Vne
Roll the helicopter to about 70 degrees of bank
Pull back hard on the cyclic

(Basically it can be described as trying to fly a Red Bull race in a helicopter)

What would happen?

I can't provide references specific to this manoeuvre. The main texts on helicopter dynamics only generalise since such a manoeuvre isn't flown in real life or tested during certifications.

When you raise the collective, you increase the pitch of the all of the blades together.  In most flight regimes, this means increased angle of attack and therefore lift.

The cost of producing lift is drag which always increases.  So, there is a direct relationship between the collective lever and power required from the engine.

As you continue to raise the collective, AoA and lift will increase until you reach a point where the power required to overcome drag exceeds the rated power of the engine.  Most helicopters permit operation beyond the maximum for a short time, e.g. 2 or 5 minutes, typically during take-off.  Continue to increase AoA beyond this limit and drag will exceed power available and the rotor RPM will drop (a reduction in rotor RPM caused by drag exceeding power through too much collective is known as "droop").

As RRPM decreases, so does lift and the helicopter will descend.  The relative airflow into the disk now has a greater horizontal component and the angle of attack will increase, either causing yet more drag or stalling the rotor.

So there is some point, before the collective is pulled fully up where maximum lift is attained.  Beyond that, lift reduces until recovery or a stall.  The only time I have ever raised the collective fully is during "full and free" movement checks with the engine off as part of pre-flight.

Your hypothetical manoeuvre:

From VNE in level flight, you roll into a 70 degree bank.

The TRT( total rotor thrust) is always acting perpendicular to the plane of the rotor disk.  Since you are in a bank, and I assume it is at constant altitude, there is a large horizontal component of the TRT to maintain the acceleration in the turn and a large vertical component to oppose gravity and maintain altitude.

This requires a lot of power which means a lot of collective and a lot of drag.  I don't know for the AH-6, maybe there's a chart relating bank angle and speed to power but let's just say, your collective is going to be somewhere near the max lift point with a manoeuvre like this.

Since you are now accelerating with max power, your speed will decrease.  Therefore lift will decrease and since you are at or near max power, you will begin to descend.

So far, this is probably recoverable simply by rolling level and reducing collective until back inside the performance envelope.

The second part is where it definitely will go wrong. In all helicopters with a non-rigid head, including the AH-6, the disk will adopt a new plane of rotation rapidly.  There is then a significant delay before the fuselage adopts a new attitude which puts the TRT perpendicular with the aircraft Z axis.

Pulling hard on the cyclic will very rapidly put the disk into a new plane of rotation which has a large angle between the plane of rotation and the fuselage.  Additionally, the disk loading will increase dramatically and the blades will be forced downwards.  On an articulated head such as that on the AH-6, droop stops will prevent the blades from bending down so much that they fold.

Hitting the droop stops a little is uncomfortable with a lot of vibration. Pulling hard on the cyclic at VNE in a 70 degree bank at or near max power may cause failure of the droop stops or blades or cause the static droop stops to contact the  mast possibly leading to rotor separation.

Let's assume that there is no failure.  The plane of rotation will be at the maximum permitted by the physical limits of the flapping hinges which is greater than the angle between the plane of rotation and the fuselage.

Therefore, I would expect a hard pull on the cyclic to result in a blade hitting the tail or the top of the cockpit.  Either results in destruction of the aircraft, either as it breaks up at that point or when it hits the ground 10 seconds later.

First post here. I'm not a pilot, but this is how I see it happening with my understanding of helicopter forces.

Collective is up a fair bit since we've been raising it.  VNE is approaching fast, and we may be starting to feel a retreating blade effect, so we might be naturally pitching left ever so slightly.  If we propogate that to 70 degrees, then quickly pull back hard on the cyclic, with the collective never changing, the main rotor will pitch up in front and down in back to an extreme, and the blades will slow down considerably with all that drag.  You will also experience inverse torque and your hull will begin to twist right abruptly unless extreme action is also taken on the rudder, throttle, and collective to mitigate the sudden change in vector momentum.  You will also pitch wildly to the left even more, which will further confuse your vector of travel, adding strain on your hull, main rotor, and drive shaft.

Imagine pulling up, tilting left, and spinning right.  Now you're sort of flying sideways/backwards, ~270 degrees from your previous travel vector, and still spinning your hull to the right.


How is a fuselage 'puncture' repaired? Operator on phone, GoAir plane hits aerobridge

How is such a 'puncture' repaired? Is the cabin pressurization also affected by such a puncture, and approximately what is the cost incurred to repair such a puncture?

The first step for repairing damage to a fuselage skin panel is to cut out the damaged area1. Cracks tend to propagate, so an area much larger than the damage will be removed to ensure that all damaged material is removed. The stiffeners that are behind the fasteners will need to be inspected for damage as well.

A filler plate will be cut to fill the hole in the skin. One or more doubler plates larger than the removed area will then be cut and all plates will be riveted together. The doublers will possibly be on both sides or at least thicker than the surrounding skin. See here for some examples.

It's likely that a larger repair such as this one will need to be inspected regularly to ensure that the repair holds strength.

It's hard to tell where exactly this damage is located, most of the aircraft skin is part of the pressure vessel. However, the airplane is not completely airtight, so a certain amount of leakage is normal. A hole of this size will probably not prevent the fuselage from pressurizing, though larger ones will.

The cost will depend on the extent of the repair, but could be from \$5,000-\$15,000 (300,000-1,000,000 INR) based on this estimate.

1Actually, the first step in repairing damage to an airplane is consulting the structural repair manual (SRM). This is provided by the manufacturer and describes general methods for repairing minor and typical damage to airplane structure. If the damage is beyond the allowances in the SRM, the manufacturer will be consulted. All of this is to ensure that the repair will meet or exceed the strength and life of the original structure, or if this is not feasible, that the limitations of the repair are documented and understood. Major repairs will require extra inspections to ensure that the structure is still in good shape. Aircraft skin will not be welded, since this can cause heat damage to the metal, requires careful skill and care to do properly, and will not hold up well to fatigue.

The outer fuselage is not the pressure hull. The pressurized cabins are a different shell and are inside the fuselage. This kind of damage would have no effect on cabin pressure. Even if the cabin hull had been rent by a hole of this size, the effect would be minor. The pressure hull is actually open to the air anyway. Pumps continuously add air to the pressurized area while air flows out through various points. The hole would have to be much larger to defeat the power of the pumps.

To make the repair two things need to be done: the larger open hole needs to be patched and the dent and abrasions around the framing member need to be corrected.

To repair the larger tear, the sheet metal worker will probably cut it out in a rectangular pattern with rounded corners. Then a backing plate will be fitted to the rear of the panel and welded or brazed or riveted in place (depending on the materials involved). Finally a patch will be made that fits into the rectangular recess exactly. This patch will be welded and/or rivetted to the backing plate. The patch will then be sanded smooth and painted. You will barely be able to tell it is there when it is done.

To repair the dent, it will first be pulled out by welding a stud onto the dented portion, then the stud will be pulled outwards. This is repeated until the dent has been removed. Then the small abrasion on the dented area and in the vicinity of the framing member will be filled in using a welding machine. The weld will be abrasively smoothed over, then everything repainted.

The cost to do the repair will depend greatly on where and who does it. In the Boston area I would expect it to cost about $3,500.

If you are interested in the details of acceptable methods of repair, the FAA provides a manual describing typical methodologies: Aircraft Inspection, Repair & Alterations: Acceptable Methods, Techniques & Practices (FAA AC 43.13-1B and 43.13-2B) .


What are these pipes on an Avro Vulcan? As you can see above, there are two small pipes next to the main engine exhausts on this Avro Vulcan(the one at Duxford). I'm curious what these pipes are/what they are for. Are they fuel dump pipes? If so why have two right next to each other?

Although it is difficult to say from old photos, the tubes doesn't seem to be visible on the prototype or early Vulcans.

Another Vulcan, XH558 started life as a B2 and are fitted with Olympus 202 engines. Looking at a photo of it there are what appear to be intakes on each engine, as seen here. I don't know what engines the Duxford Vulcan has, but according to Wikipedia the 202 was the definitive version for the Vulcan, with the exception of some aircraft which received the 301. As far as I can tell, one of the features with the 202 was that it had a redesigned oil separator breathing system.

Such features require air inlet as well as an outlet for the vented gases, could this be related to that system?

The two thin pipes next to the inner exhaust ports are indeed fuel dumping pipes. the Drogue chute is mounted in the rear of the fuselage directly below the rudder.


Are fighter jet cabins pressurized? I heard once that fighter jet cabins are NOT pressurized, but the pilot's suit is, which prevents problems with decompression if the cabin gets shot.

But a military fighter pilot much later told me that the cabin is, in fact, pressurized and that getting shot (the cabin) and the resulting decompression are not that much of a bother, at all.

Cabins of fighter jets are in fact pressurized, but not same as the passenger airplanes. Cabin pressure in a passenger airplane will increase (almost in a linear fashion) as the airplane climbs. However, fighters keep ambient air pressure to a certain altitude. It maintains this pressure until a certain pressure differential is met, and then cabin altitude increases to maintain this differential.

Therefore at higher altitudes, cabin pressure in a fighter jet is low enough that supplemental oxygen is required for the pilot(s). This is supplied via On-Board Oxygen Generation Systems.

By not pressurizing the cabin completely reduces stress on air frame and is beneficial during an unplanned decompression.

By the way, your friend (military fighter pilot) is correct.

Pressurisation is required in fighter pilot because as per altitude increase pressure reduces so it is required to maintain the cabin pressure relative to ambient pressure...


Which military aircraft does this seem to be? [F-16 vs MiG-21] Can someone identify the aircraft based on the components, riveting style, or other technical details visible in these crash pictures?

a) Based on publicly available details, the chief suspects are MiG21-Bis or F-16. Are there US vs Soviet construction details, materials, etc that can enable identification based on these images?

b) There is a drop-tank in the images, which is claimed to be only compatible with one of these two aircraft. I'm looking for someone familiar with the MiG-21 or F-16 to comment.

Background:

Three aircraft crashed in the Kashmir region during hostilities on 27th Feb 2019. One was an Indian MiG-21bis, whose pilot ejected and was apprehended by Pakistani locals. Another was a Mi-17 helicopter, which crashed "due to technical reasons" killing six persons onboard, within Indian territory.

No information is available about the third aircraft; Indian sources claim they hit an F-16 while Pakistani spokespersons said they did not have F-16s in the area.

These are purportedly pictures of the downed third aircraft. This could be part of an F-16, JF-17, MiG-21, SU-30MKI, or Mirage in Pakistani or Indian service.

source: Photo/ANI

source: Photo/ANI

There is also a low-resolution video available via twitter, showing the droptank recovered from the site being loaded into a truck.

Link to low-resolution video on Youtube

source: @MdShafiKhatana/Twitter

Edit:

This is a recent incident. Some sources claim the image shows a part of an F-16 engine. Is this plausible?

source: Photo/ANI

Edit 2:

Is it plausible that this number "J0269" (from the above picture) can be used to identify the aircraft as an F-16B Block 20 MLU with tail number J-269 using this link?

It's a MiG21. There was another photo showing the front section area and the unmistakable nose cone of the MIG21.


Which one is considered the departure time of a commercial flight? I was wondering, the publicly announced departure time of a commercial flight, is it the ETOT (estimated takeoff time) or is it the EOBT (estimated off-block time)? Or maybe some other time?

The scheduled time is the EOBT. Flights that depart within 15 minutes of it are considered on-time.
Standard taxi times for each airport are included in the scheduled flight time.

In this document you will find some of the many timestamps used in CDM.
As you can see there, the EOBT is the timestamp displayed in the flight plan.

The time on the ticket usually corresponds to the ETA/ETD which in term should be STOT (scheduled take-off time).

Are you extracting information from a data source or are you just curious?


Is it true that airlines won't let pilots drink diet soda? I watched a documentary called Hungry for Change that claimed pilot associations advise avoiding diet soda due to the aspartame sweetener inhibiting motor skills and killing brain cells.

Is this true?

I've never gotten anything to this effect from my airline nor pilot union, and they'd better start stocking cans of unsweetened Snapple if they were to push this, because I don't like coffee & I most certainly don't want to be drinking lots of sugary sodas to get my caffeine fix in the morning!

There are many pilots particularly from the 1980s and 1990s that have spoken about their experiences of adverse reactions through ingesting aspartame in diet products.
In those days a warning was put out that consuming these products do, in some people, cause side effects.  The General Aviation News reported "pilot George E. Leighton experienced blurred vision so severe he was unable to read instruments on his panels and narrowly avoided a tragic landing after consuming two cups of NutraSweetened hot chocolate prior to his flight." Other pilots who have lost their medical licence because of seizures, tremours and other debilitating effects, have also given warnings about aspartame ingestion.  Pilots such as Captain Harold Wilson, Pilot Haynes Dunn, Texas pilot Charles King and Captain Jim Sells. More recently, you may remember Captain Clayton Osbon flying JetBlue who had a psychotic episode during a flight in March 2012.  He was ridiculed, vilified and his character destroyed for a mental episode that was completely out of his control.  He was known to consume diet products on a regular basis and in fact actually promoted these products.  He is now suing the airline for not pulling him from the flight after he showed obvious signs of being mentally distressed before he boarded the plane.
Aspartame was passed for human consumption in 1981, under a cloud of fraud and deception with Ronald Rumsfeld at the helm of the company G D Searle who discovered the chemical concoction.  It consists of aspartic acid, phenylanaline (two amino acids) and methanol.  Each of these molecules are toxic in their own right. I would recommend that pilots and crew who ingest diet sodas, chew gum, put equal or other aspartame powders in their drinks or sprinkle aspartame powders on their food, beware. Look out for adverse reactions such as headaches, tremours, anxiety, confusion, vision problems, depression, insomnia, dizziness, memory loss, seizures, palpitations to name a few. There are 92 recorded side effects. If you get any of these effects, try stopping aspartame and see if the effects disappear. I have been researching aspartame for a long time now and apart from the dubious way it was passed for consumption, the chemical makeup of this molecule can cause problems for some people. Unfortunately, the warnings that were once given out to pilots, have now been forgotten but it is obvious that pilots are having problems either through fatigue or fatigue exacerbated by aspartame consumption. I can't believe that all the 'pilot errors' that are reported with accident investigations are because of incompetent pilots.  There's something else going on.  If you're a pilot, perhaps it would be best to err on the side of caution and avoid this sweetener.  By the way, it won't help you get slim either!
From 1994-2014, there have been 344 commercial aircraft incidents (16,490 fatalities).  Out of those, over half have been put down to pilot error and nearly 60 to human error.  If there's just a chance you may be affected by this chemical, wouldn't it be prudent not to consume it?  If just one of those 344 incidents was because of aspartame, it's not worth taking the risk that you may have an adverse reaction whilst in the cockpit. Ches Power


What do the regulatory bodies say about landing in CAT IIIB? I typically read that CAT IIIB approaches must be flown by an autopilot to touchdown. Looking for a reference (regulatory bodies* in particular) to track the statement only leads to company SOPs.

In other words, with the runway in sight at or above the DH, is it written somewhere (excl. SOPs) that the pilot flying can/can't take manual control when the approach was started in CAT IIIB minima?

Full disclosure: I completely understand it's unwise and unsafe to even suggest hand flying an approach when that's the view:

RVR 200 m. (wikimedia.org)

* FAA, EASA, and ICAO, are acceptable (I couldn't find anything in ICAO Annex 6).

The ability to perform autoland approaches requires specific approvals.  In the US, it is outlined in AC 120-28D Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout. BTW, CAT I and CAT II approaches are covered by AC 120-29 Criteria for Approval of Category I and Category II Weather Minima for Approach.

The AC describes the necessary installed equipment, specific crew training requirements, and the need for operational procedures.  Once approval is granted, the crew training and operational procedures become part of the airline Op Spec.

If any hand flying is allowed, it will be spelled out in the Op Spec. Since Op Specs do vary from airline to airline, it's not possible to state definitively what is or is not allowed.

That said, it is physically possible to manually take over and land the aircraft. If the crew has the runway in sight at 200 feet, it's really no different that a CAT I approach. At 50 feet on an autoland approach, the autopilot will be starting the flare and de-crab maneuver, so manual intervention at this point may or may not be allowable depending on the specific aircraft performance and the airline's operational philosophy.

A fail-operational autoland system would be able to continue the approach for any single failure below alert height (between 100 and 200ft RA for most aircraft). If autopilot would completely disconnect below alert height, it would mean that a multiple failure has occured.

This kind of scenario would be covered by company SOPs, however I doubt any sensible company would allow continuation of a Cat IIIb approach after complete autopilot failure. We are talking about hand flying an aircraft in RVR as low as 75m. Not the best idea.


How much energy is "wasted" in an aeroplane's descent? In descent, an aircraft must lose both potential energy (in order to get down to the ground) and kinetic energy (in order to land at a safe speed).

Weight and drag can take care of that. An unpowered plane will eventually slow down and sink. However, sometimes it won't happen fast enough without additional control input, and the pilot must act to make the plane less aerodynamically efficient, and dispose of some surplus energy that would otherwise be retained longer.

If a plane completed its descent from top to bottom without using the control surfaces to:

dump excess lift
bleed off excess speed along the way

until actually landing, that would mean no waste of this energy in descent: all of it was used productively to get the aircraft to its destination, and none of it needed to be thrown away to get the plane low enough and slow enough fast enough.

In a particular descent, would it be possible to calculate or estimate how much of that energy in practice does in fact get "wasted", i.e. needs to be deliberately thrown away?

A meaningful numeric value is going to be hard to calculate, because so much depends on the aircraft you're working with.  In practice, if I can pull off power at the calculated Top of Descent point, descend, meet any ATC crossing restrictions, fly the approach, configure as late as possible, and push up the power right at the point where I'm required to be stabilized for the approach, then I've flown an efficient approach & wasted none of the energy that I started with.

If I have to add power sooner than that, I started down too soon (or wasted energy in some way); if I have to deploy drag (which includes configuring early), then I started down too late & so I have excess energy that I need to dissipate.

I suppose "amount of time speedbrakes were up" would be one way to score how much energy had to be wasted; add to that how long flaps & gear were out before they were "needed" (given the requirement to be stabilized & configured at a given point in the approach).

Here's the complication, though.  If I see early on that I have too much energy, I can push the nose down & gain speed, which increases the parasite drag of the aircraft -- thus dissipating energy.  If you have enough time at the higher speed (not just push the nose down, gain speed, pull the nose up, lose speed -- that doesn't accomplish very much), then you can get rid of excess energy without having to deploy speedbrakes or configure early.  And the reverse, if my standard descent is at 280 knots (above 10,000', obviously) but I slow up to 250 knots early enough, I can "gain" some energy, since I'm losing less with less parasite drag.  How you could account for all of that in a "score" is beyond me.

In the ideal case, I can descend to glideslope intercept altitude, level off there, let the speed decay, then at the point when I need flaps to fly any slower, I extend the first increment of flaps, let the speed keep dropping off, start down the glideslope (speed decreases more slowly or if the aircraft is light enough, not at all), at about 500' above the "stabilized approach" gate I'll lower the landing gear & extend more flaps, so that as I approach that gate I'm extending the last increment of flaps, my speed reaches VTarget, and I add power.  No extra time with power above idle, a fuel-efficient approach.

ATC may not let me do that, though; they may keep me fast until close to the runway, in which case I'm extending flaps & gear closer to their maximum speeds so as to dissipate more energy more quickly and still be configured & stabilized at the appropriate point.  (The 737 dissipates speed pretty quickly fully configured at idle power, even descending on the glideslope.)  So did I "waste" some energy in that scenario?  Yes.  But keeping ATC happy so they can get efficient runway utilization is a worthy goal as well.

Sure.

Let's take the simple case where mass is assumed to stay constant during the descent.

The formula for kinetic energy is KE=(1/2) * m * (v^2), where m is mass and v is velocity.

Compare the kinetic energy at start and end of descent, using True airspeed.  If True airspeed is constant, Kinetic energy is constant.

Now compare the (gravitational) potential energy at start and end of descent.  Potential energy = mgh, where h is elevation and g is the gravitational constant and m is mass.

The energy lost is the decrease in potential energy, minus any gain in kinetic energy.  Or the decrease in potential energy, plus any loss of kinetic energy.

If airspeed is constant throughout the descent, we are losing ALL of the potential energy we had at the start of the descent-- NONE of it is being converted to kinetic energy, or to any form of stored energy.

Too bad we aren't coming down by using a prop as a windmill to power up a battery as well as create drag, rather than by opening spoilers and such.

Alright, expanding this answer to consider comparing the energy lost in the actual descent to touchdown, to the energy lost in in some representative "ideal" maneuver.

What is the "ideal" maneuver?

Ideal Option 1)  -- at the same point in time that the descent would be normally be initiated, we instead smoothly pull back on the yoke to transition to the angle-of-attack that gives the max ratio of lift/ drag (unless this would arc the flight path up beyond vertical unto the second quarter of a loop, then we have to use a slower rate of increase of angle-of-attack to pull less "G".)  Anyway we end up doing a (possibly rather steep) "zoom" climb to gain altitude and lose airspeed and then we let the flight path arc back down into a steady-state glide at the angle-of-attack (and airspeed) that gives the max ratio of lift/ drag.  As we overfly the point where we'd touch down with the real-world method, we note our altitude and airspeed; this gives us a basis to compare the actual energy lost in the real-world method with the energy lost by the "ideal" maneuver.

2) Ideal Option 2-- we roll back time to the point where we can glide all the way to touchdown with the plane in a clean configuration, at the speed that gives the flattest glide, after initiating a highly efficient transition to the best-glide speed such as that described in Ideal Option 1.  At the moment of touchdown, compare the fuel in the tanks at the moment of touchdown with this Ideal Option, to the fuel in the tanks with the standard method of a longer cruise period followed by a steeper shorter descent. Also consider the difference in airspeed at the instant of touchdown.  Now we have a basis to say how much energy is "wasted" with the standard method (but how do we evaluate the energy stored in the fuel?  Do we consider ALL the energy stored in the chemical bonds, or just the energy that the engine would actually be able to extract?)

3) Ideal Option 3-- instead of descending we just maintain cruise speed and altitude so we lose neither kinetic energy nor potential energy.  So no energy at all is expended with this ideal option.  Well, that's a little silly and shows that we really do need to take into account the potential chemical energy we're taking out of the fuel tanks, both with the actual method and with the ideal method.

Since we're now considering the energy stored in the fuel, we now ought to revisit the ideal option 1.  How much power should we carry?  Just enough that the engines create zero thrust-- keep the fans turning under power so we're not using them as giant windmills?  Maybe just a little less than that?  Surely we don't want to chop the fuel flow to zero do we?  Then the fans would make way too much drag.

On the other hand if we are considering, say, the energy that could be released by the fuel in a nuclear fusion reaction, then we better leave it all in the tanks for the "ideal" descent option, as it is too precious to waste any.

While we're at it we should really note that with the real method-- and arguably with the ideal methods too -- all the kinetic energy that we're carrying at touchdown is wasted-- for the real-world method at least I guess we really need to measure all the way to the point where the plane comes to a stop with zero energy.  Also consider all the fuel wasted as we apply reverse thrust.  None of this comes into play for the "ideal" methods; we get to use the most efficient method of flight possible to keep the plane moving through the air till we are overflying -- the touchdown point?  The point on the runway where the plane would come to a stop in the real world?  The gate at the terminal?  With the no-landing ideal methods such as #1 and #3, as we're about to overfly the gate at the terminal, high above it, do we get to ease the yoke further back and bleed off airspeed all the way to stall speed, helping us to maintain altitude for a while without burning (much) fuel, or even to gain some altitude?  Sure, why not.

Lots of different ways to set up this problem.


Can this helicopter/wrecking ball combo remain airborne? I just ran into this meta-answer which includes this image:

My initial guess is that the wrecking ball is far too heavy for the helicopter to lift, but I'd be interested in the numbers to see just by how much.

From this silhouette, can the make/model of helicopter be determined?  (I'm hoping this image is based of some real-life craft)
If so, presumably the vehicle length is known, and the diameter of the wrecking ball may be calculated (spikes may be ignored) according to the scale of this image
Assume the wrecking ball is solid mild steel with a density of 7.85 g/cm³.  The mass of the wrecking ball can then be calculated
If the make/model is known, then presumably the max payload weight is also known, and we can see if staying aloft is a possibility or not.

Close-voters: I asked this question on Aviation.SE because the key piece of information I was missing was the identification of the craft in the cartoon image.  I think aircraft-identification is on-topic here.

EDIT

Huge apologies to one and all, in my first version of this answer I mistyped 184 as 84, and then proceeded to do all my calculations based on that.  This is the corrected version.

Also, huge thanks for all the up-votes and comments. I'm so glad it's not just me that is enjoying this question immensely.

The radius of the ball in the image is 21 pixels. The length of the fuselage is 184 pixels, which is 50' 7.5" (15.43m) according to widely available statistics. Hence, a pixel represents approximately 84mm

So, the radius of the ball is 21 x 84 = 1764mm = 1.764m

And the volume of the ball (if solid) is 23.0 m³.

For it to be liftable by the helicopter, using Pilothead's quote of 9000lb (4082kg), the density would need to be 177.5 kg/m³. Mild steel, at 7,850kg/m³, is way too heavy. Expanded polystyrene has a density of 28-45kg/m³, so that would be feasible.

However, if the ball were not solid, and were made of mild steel, the total volume of steel permissible would be 0.52m³. At an outer radius of 1.764m the inner radius must be at least 1.750m to keep the volume of metal down to 0.52m³, so the ball would only be 14mm thick (9/16") and would only cost about $3,300.

So, let's try Titanium (darling of the aircraft industry). With a density of 4,506kg/m³, we could have a volume of 0.90m³. That would allow a thickness of 24mm (15/16"). However, the price of that much titanium would be $229,500 (today's prices - 6 years ago, it would have been over a million US dollars).

Aluminium, however, has a density of 2,700kg/m³. So now we could have a volume of 1.51m³, allowing a thickness of 40mm (1 9/16"), and would only cost $9,360

Pumice, at 250kg/m³, would allow a volume of 16.33m³, allowing a thickness of nearly 0.6m (23½"). That would be very sturdy, I reckon. I can't find a price for a solid lump of pumice that size though.

Incidentally, the people in the picture are also about 21 pixels tall, making them around 1.76m (5'9") tall.

This has now taken up about 3 hours of my time, which like all other hours up to now, are hours I'm never going to get back.  However, it has been so much fun.

FURTHER EDIT

Following a suggestion from Jules in the comments, I've discovered Lithopore 75-150, which is a ridiculously low density foamed concrete.  This could give us the magical solid sphere that we want, with sufficient strength to squish multiple personnel without significant damage to itself.  At the 75 kg/m³ end of the range, we could possibly even have some spikes, and a non-negligible cable mass too.

More maths and engineering research required.....

The image is fake.  There are no known slingloads for the UH-60 which correspond to the image shown.