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Slip Sliding Away

Know what to expect on arrival before you depart,

and then calculate accordingly.

By Patrick R. Veillette, Ph.D.

ll

Citation e 500 pilots were making a

VOR approach to Runway 32 at Sault

Sainte Marie.After breaking out of the

clouds at 2,500 feet and aligning the airplane

with the runway, the captain noticed

"maybe compacted snow or maybe ice with

fresh snow" on the runway - not surprising conditions in northern Michigan in

February. When the aircraft was on a two -

mile final, a voice came over the Unicorn

frequency and said that braking action was

nil. This observation gave further weight to

a NOTAM in effect at the time that stated,

"icy runway, nil braking" at the airport. The

captain told the copilot they would perform

a go-around if by midfield the aircraft was

not decelerating adequately.

The captain touched down within the

first third of the runway as planned, but as

the Citation neared the midfield point, the

airplane fishtailed. Passing midfield, the

captain called for a go-around as he added

power and retracted the airbrakes.

"There is not enough runway!" the first

officer called out and then braced.

Moments later the twinjet ran off the end

of the runway and down an embankment.

After sifting through all the data surrounding that 2001 mishap, the NTSB determined the pilot exceeded the available

runway distance during landing and delayed

executing a go-around. Contributing factors in the accident were the pilots' improper infligh t planning and decision

making regarding the contaminated runway,

the pilot's disregard of the airport

NOTAMS and the Unicom warning, and,

of course, the icy runway.

Surely those of us who have been confronted with nearly identical conditions -

and that's probably quite a few - are quietly saying, "It could have been me." It's a

harrowing feeling to be skidding down the

runway with brakes and thrust reversers

that seem completely ineffective. And it's a

feeling many have known too well.

A Flight Safety Foundation study of business jet accidents between 1991 and 2003

found that 59 of the 104 landing accidents

www.AviationNow.comjBCA

and all 176 reported landing incidents involved overruns, with roughly 70 percent of

those involving skidding off the end of a

contaminated runway. A separate analysis of

FAA accident and incident data for years

1978 to 1993 uncovered 769 instances of aircraft overruns and excursions in which ice,

snow and slush were contributing factors.

When reviewing weather reports for a

destination, it's natural to concentrate on the

ceiling and visibility to determine if the field

is above minimums. But the runway's surface condition deserves close scrutiny as

well.

That said, the information is often unavailable - especially if the facility is a general aviation airport without an operating

control tower. Even in the rare circumstances when braking reports are available,

their reliability is often questionable. A report of "fair" braking from the pilot of an

aircraft with dual wheels all around, operable anti-skid and plenty of tire surface for

added braking capability may not necessarily be representative of the braking condition for an aircraft with single-wheel

gear whose anti-skid is inoperative or nonexistent.

Business & Connnercial Ariatio» • November 2005109
Safety

Table 1

Airport Conditions Frequently Cited

in Business Jet ASRS Reports, 1991 to 2003

Inadequate runway conditions, maintenance 33%

Poor field maintenance (lights, surface condition, paint, markings) 28%

Poor ramp conditions (torn up, congested, narrow taxiways) 26%

Lack of weather reporting (no METARs and/or TAFs) 19%

Lack of adequate runway condition reports 18%

Lack of adequate snow removal 14%

Note: Some reports contained more than one category.

Source: NASA ASRS

In fact, getting accurate information regarding the runway's surface is rather

problematic. As part of the multi-year research behind the Flight Safety

Foundation study, I reviewed a sample of

287 NASA Aviation Safety Reporting

System (ASRS) reports submitted by pilots of business jets who cited problems

involving runway surface conditions ranging from poor maintenance to bad lighting to inadequate condition reports.

Obviously, any time the runway surface

is anything but dry and clean, braking effectiveness is going to be reduced. The

presence of a fluid contaminant such as water, slush, loose snow, deicing or hydraulic

fluids or a solid contaminant like packed

snow, ice, and sand reduces the friction

force between the tires and the runway surface. The contaminant can also create a

layer of fluid between the tires and the runway, increasing the probability of hydroplaning. And since main and nosewheels

can be affected, hydroplaning not only

compromises the aircraft's braking abilities,

but its directional control as well.

Of the 59 business jet overrun accidents

uncovered, 19 involved rain-covered runways, as did 56 of the 176 incidents. Snow,

slush and ice were factors in most of the

rest.

In an ideal world, pilots would ascertain

runway conditions from accurate reports

before every flight. Naturally, they would

prefer the destination airports to have good

clear, well-maintained runways stretching

beneath cloudless skies. In the unhappy

event that rain is forecast, then at least the

runway has a porous friction course (PFC)

surface or is "grooved."

In the real world, however, weather happens and runways get fouled. That means

when evaluating the suitability oflanding at

an airport pilots need to consider both the

runway surface and what nature may have

set upon it. For example, a forecast of mod110 Business & Commercial Aviation. November 2005

erate or light rain on a runway with a PFC

or with grooves under most circumstances

would normally be just a "wet" runway.

To the Europeans, at least, that means

something quite specific. TheJAA defines a

"wet" runway as one covered with water or

having sufficient moisture to make the surface appear reflective, but without significant areas of standing water.

PFC or grooved surfaces tend to hasten

the drainage of rain from the runway's surface and inhibit the build-up of puddles.

During a summer deluge, these surfaces

shed standing water in about 15 minutes

(that is, unless a strong crosswind inhibits

drainage on the upwind side.) Some 800

runways in the United States have transverse grooves. Both PFC and grooved surfaces are common at airports with airline

service. Unfortunately, many general aviation airports lack these features and, thus,

moderate or heavier rainfall there can result

water puddles, leaving the runway "contaminated." Here again the Europeans have

specific conditions in mind. To the JA!\, a

"contaminated" runway is one on which

Table 2

Runway Conditions

During Overrun Events

Business Jet Accidents and Incidents,

1991 to 2003

Accidents Incidents

Dry 17 53

Rain 19 56

Snow 12 36

Slush 6 17

Ice 5 14

Total 59 176

Source:NTSB. U.K. Air Accident

"more than 25 percent of the runway surface area is covered by surface water more

than 3.0 rnm (0.125 inch) deep; by slush or

loose snow; by snow, which has been compressed into a solid mass which resists further compression and will hold together or

break into lumps if picked up; or by ice, including wet ice."

By contrast the FAA eschews such precision, and considers a runway contaminated

"whenever standing water, ice, snow, slush,

frost in any form, heavy rubber, or other

substances are present."

-i.. Part 135requires operators to add 15 peri ~ent to their runway length requirements

whenever the runway is "wet." However,

several authorities recommend increasing

the stopping distance by much larger factors than that. A handout I received at

FlightSafety International during my last

simulator training states that a wet runway

increases the stopping distance by 25 percent to 100 percent.

According to the "Advisory Information"

in the FAA approved flight manual for a

Cessna Citation: All flight manual field

length data assumes a dry, hard surface runway except where otherwise noted. Precipitation covered runway conditions will degrade

braking effectiveness and will require sif512ificantly greater actual takeoff abort and landingfield lengths ....

Correction factors for adverse runway

conditions are presented below. The dry

runway landing distances are to be multiplied by the correction factors:

\-Vet Runway -less that 0.01 inch water

- Correction Factor: 1.45; Water -less

than 0.5 inch water- Correction Factor:

2.20.

The Dassault Falcon 900 Aircraft

Operating Manual goes even further than

the JAA. and differentiates between contaminated and slippery unways. Dassault

defines a contaminated runway as "a runway where more than 25 percent of the runway surface area, within the required length

and width being used, is covered by standing slush or snow more than 3 mm (0.125

in.) deep. Patches of snow, ice, and/or slush

on the takeoffllanding portion of the runway are also considered runway contaminates. Braking action, if reported, is fair or

good." In these conditions, the Falcon 900

AOM stipulates that a factor of 1.50 should

be added to the landing distance.

Dassault defines a slippery runway as one

on which "standing water, snow, ice patches,

and/or slush cover 50 percent or more of

the takeoffllanding portion of the runway.

Braking action, if reported, is poor." In the

case of a slippery runway, the Falcon 900

AOM stipulates that a factor of2.00 should

be added to the landing distance.

Additionally, the Flight Safety

www.AviationNow.com/BCA
Safety

Foundation's Approach and Landing

Accident Reduction (ALAR) briefing notes

prescribe the following landing distance factors: Wet runway -1.3 to 1.4; Standingwater or slush-contaminated runway- 2.0

to 2.3; Compacted snow covered runway1.6 to 1.7; Icy runway- 3.5 to 4.5.

By the way, all of these correction factors

assume the anti-skid system is fully functional. Obviously, operations on contaminated runways with either inoperative

anti-skid or thrust reversers should be minimized unless absolutely necessary and even

then only with all other factors within suitable safety margins.

While these correction factors are useful

for determining the additional runway

needed, it's important to note that they assume the runway composition, condition

and construction, the amount of precipitation and the depth of main landing gear tire

tread are all within satisfactory limits. And

as we all know, those circumstances do not

always exist, increasing the required landing distance even more.

The Citation Operating Manual further

advises that, "With precipitation cover on

the runway, braking should be very judicious. If runway length permits, delay braking slightly until some aerodynamic

deceleration has taken place. Under normal

braking conditions the anti-skid system is

very effective in preventing skids and proAerobraking

It looks great in the T-38, but it doesn't work for business jets

It was the copilot's leg in his relatively new career after flying

fighters for Uncle Sam. During his approach briefing he confidently mentioned he would use aerobraking to bring the jet

to a stop. The captain wasn't familiar with this landing technique but assumed the copilot with his "right stuff resume"

knew his business. After touching down, the copilot held the

nose up, something the captain hadn't seen before. At first it

just seemed unusual, but then as the jet failed to slow appreciably while rapidly traversing long lengths of pavement,

the captain grew increasingly concerned. Finally, with the runway's end approaching, he took control, but too late and the

jet slid offthe end of the runway. It was an inauspicious way

to begin - or possibly end - a civilian flying career.

Unfortunately it isn't the first time this has happened.

Aerobraking has been mentioned at a recent aviation gatherings as a proper method for reducing the number of approach and landing accidents. However, the procedure goes

against the findings of extensive accident studies and

recommendations.

One ofthe problems noted in the Flight Safety Foundation's

recent business jet study was the frequency of overruns, a

significant number of which were attributed to incorrect braking techniques including aerobraking. Other studies have

noted this problem as well. According to an in-depth review of

76 approach and landing accidents worldwide between 1984

and 1997 by the Flight Safety Foundation's Approach and

Landing Accident Reduction (ALAR) Task Force, "one of the

primary factors affecting braking in runway excursions or runway overruns has been a reliance on the incorrect technique

of maintaining a nose-high attitude after touchdown to achieve

aerodynamic braking."

Where did this "aerobraking" concept come from? Target

the Talon. It turns out that many of the pilots who rolled to

grief using aerobaking had learned to fly in the North American

T-38, long the U.S. Air Force's primary jet trainer, or similar aircraft whose small thin wings were built for one thing: speed.

The disadvantage of such designs was they required very high

landing speeds. It's been over 20 years since I flew the Talon,

but I seem to remember that we'd touch down at 160 to 175

knots with full flaps, and about 15 knots higher without flaps.

As student pilots we were admonished to keep our feet off

of the brake pedals above 100 knots lest we inadvertently

blow a tire. So how did we slow the T-38 upon touchdown?

We held the nose of the aircraft up to a relatively high pitch

112 Business & Commercial Aviation. November 2005

attitude, letting the profile of the jet create enough added

drag to gradually slow the aircraft to a range where we could

let the nose down and begin wheel braking. Aerobraking.

The F-16 and F-15 pilot community continues to use this

method. I understand there's informal competitions within

some squadrons to see who can keep the nose of the aircraft

off the runway the longest. This of course leads to the elevator running out of airflow and the nosewheel slamming down

onto the runway, sometimes with ill effect and much to the

consternation of the maintenance troops.

Pilots in mufti who try to transfer this technique to the civilian world fail to account for the significant design differences

between the average business jet and the supersonic Talon.

These differences include aerodynamics, aircraft systems and

momentum, all of which dictate an entirely different method

for dissipating the energy during landing. The wing surface

area alone of the average business jet is significantly grater

than the T-38's. Moreover, the T-38 rolls on small single tires,

and has no spoilers, no anti-skid and no thrust reversers.

Meanwhile, many business jets have dual bogey configurations with wide tires that have a bigger foot prints and have

anti-skid and thrust reversers.

One of the most overlooked and understated recommendations in the Flight Safety Foundation's ALAR manual is this

potent statement: "The importance of timing during the use

of all means for stopping the airplane cannot be overemphasized. As soon as it is definitely known that the main wheels

www.Avlationrvow.corn/Bcn
ducing minimum stopping distances; however, on a precipitation-covered runway, the

phenomenon of hydroplaning may greatly

reduce the anti-skid effectiveness, due to the

possibility of the airplane wheels not rotating up to a speed equal to the airplane's

ground speed."

Since hydroplaning may occur at touchdown, this prevents the wheels from spinning and from sending the wheel-rotation

signal to the anti-skid system. The Flight

Safety Foundation's ALAR program recommends a stabilized approach with a finn

touchdown in the touchdown zone to increase the probability of wheel spin-up and

to minimize the chances of hydroplaning.

The degree of hydroplaning depends on

the runway's design, the depth and type of

contaminant, tire inflation pressure, ground

speed and anti-skid operation. Good tread

depth tends to relieve hydrodynamic pressure under the tire on wet runways. Proper

inflation is important because a low tire

pressure lowers the minimum hydroplaning speed. Anticipated operation on precipitation-covered runways dictates close

monitoring of tire condition and pressure.

Pilots should pay particular attention to

adverse wind conditions when there is a

possibility of contaminated runway conditions. The risk of hydroplaning is directly

increased with a tailwind due to the added

speed component and so landing on preare rolling on the runway, use elevators to bring the nose

wheels onto the runway smoothly and hold them there.

Immediately raise speed brakes to their full deflection, apply

wheel brakes and actuate thrust reversers."

While that observation and recommendation may seem obvious, failure to embrace them have left too many airplanes

in the dirt and left their professional pilots looking like anything but.

According to flight test data from Boeing, the most important factor that affects total landing distance is the coefficient of friction between tires and the runway surface. This

coefficient results from many variables, such as tire tread design, runway material and surface condition, air temperature

and rolling speed of the tires. One of the primary factors governing braking effectiveness is the amount of weight on the

wheels, which is directly controlled by aircraft attitude and

speed brake deflection.

Some pilots believe that holding the nose off the ground results in substantially higher drag on the aircraft. Yet when

test pilots (many of whom also spent their formative years in

the T-38) tried this rollout maneuver on jet transports, the

added drag was not significant, and the side effects were potentially threatening. Specifically, the higher angle of attack

develops lift and prevents the brakes from working to their

full capacity.

Additionally, the Flight Safety Foundation's ALAR study

showed that lowering the nosewheel to the runway puts 60

percent of the aircraft's weight on the main wheels, whereas

keeping the nose in the air puts negligible weight on the main

wheels. This fact is reiterated in training literature published

by Boeing, and earlier by McDonnell Douglas.

Furthermore, some aircraft have nosswheel spin-up transducers which are required for optimum operation of the antiskid. Therefore, the ALAR task force recommends that

immediately after touchdown, the nosewheel should be lowered to the runway and held there positively by forward elevator pressure until taxi speed is reached. (In crosswinds

keeping the nosewheels on the runway noticeably improves directional stability, especially on contaminated runways.) The

task force also noted that extending ground spoilers results

in 85 percent of the aircraft's weight being placed on the main

wheels plus a 130-percent drag increase from spoilers.

Deployment of these lift-destroying devices is so effective

that many transport aircraft now have ground spoilers that

extend automatically as soon as the air-to-ground sensors detect weight on wheels,

Some aircraft have spoiler systems that are even more efwww.Avlatlonlvow.c orn/Bt.A

fective than simpler ground spoiler designs. Anyone who has

a Hawker can attest to the wonderful effectiveness of the

"lift dump" system, and specifically the deceleration felt as

soon as the flaps extend into the lift dump position. It may

not quite compare with the rapid deceleration of catching a

"three wire" on a carrier, but there's no question that it gets

you stopped fast.

Deployment of these lift-destroying and drag-creating devices is even more important on a contaminated runway.

Spoiler deployment is so important the ALAR report recommends that the pilot-monitoring callout the spoiler extension

after touchdown.

While nothing beats tile feeling of "greasing" a landing, the

fact is that a firm landing is better. Planting the gear ensures

wheel spin-up for the anti-skid system and in some aircraft for

the extension of spoilers. Once the main wheels spin up and

brakes are applied, the anti-skid system determines a braking threshold. Full pedal depression requires less time for

brake modulation than lower pressure. Changing the brake

pedal pressure requires time for the new control level to be

established, thus increasing the landing distance. So for maximum effectiveness, apply the brakes steadily to full pedal deflection as soon as the ground spoilers are deployed and main

wheel spin-up occurs.

Thrust reversers should be applied early at maximum power.

Avoid using differential reverse thrust and if one reverser fails

to deploy immediately, its opposite should also be left at the

interlock. Due to the three- to five-second delay before the

build-Up of full effective reverse thrust, brakes will normally

be operating before reverse thrust. The application of brakes

tends to counteract any adverse pitch-up tendency that may

develop. This is yet another reason reinforcing the importance

of the timing of proper braking control inputs. Lastly, as a general rule, it is recommended that braking continue until assured that the aircraft will stop within the remaining runway

length.

Many of us would love to return to those early years when we

swagger out to the sleek T-38, don our "speed jeans" (a.k.a., gSUit), strap on and in and then feel the kick of those burners during takeoff. But those days are past as are many of the techniques

that applied. Professional pilots are expected to use tile procedures prescribed by the manufacturer's AFM. Use of non-standard

techniques, especially during critical phases of flight, can result

in bent metal, and an assessment by tile FAAthat the pilot - a

right stuff resume notwithstanding - had been "careless and reckless" and wrong. B&CA

Business c: C0777717crciil!.'-l'L';m;oJ]. ;Voi.H'777beT ~005 113
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114 Business & Commercial Aoiation • November 200 5

Safety

cipitation-covered runways with any tailwind component should be avoided.

Adverse winds on a contaminated runway

can also make directional control dicey at

best. Some manufacturers publish crosswind limitations that are very restrictive .

For example, the Dassault Falcon 900 performance manual restricts the maximum

safe crosswind on an icy runway to a mere

five-knot component.

Flight crews should not delay lowering

the nose to the runway and quickly extend

the ground spoilers/lift dump. This action

increases weight-on-wheels and, consequently, brake effectiveness. According to a

handout from my simulator training session, inoperative spoilers can add 25 percent

to the runway requirement. Pilots should

use maximum reverse thrust as soon as possible after touchdown since its effectiveness

is greater at higher speeds. Thrust reservers

will decrease the stopping distance on a slick

runway by 10 to 30 percent.

According to Boeing research, excessive

cycling of an anti-skid system reduces total braking effort. Maximum braking occurs when only enough brake pedal

pressure is used to produce an occasional

anti-skid release, or about one release over

two to three seconds. You can feel this antiskid cycling as a "kick" in the brake pedals.

Light pedal pressure can produce excessive

cycling. The pedal pressure that keeps the

cycling at the minimum rate provides the

best braking.

The tires use friction to create a turning/cornering force, and the reduced friction on a contaminated runway

compromises the aircraft's directional control. Maintain control through the rudder

pedals. Use of nosewheel steering above

taxiing speed may cause the nosewheel to

hydroplane and result in the loss of nosewheel cornering force, with a consequent

loss of directional control.

When differential braking is necessary,

apply pressure on the required side and but

not on the opposite side to regain directional control. If braking is not completely

released on the opposite side, brake demand

may continue to exceed the anti-skid regulated braking, and thus no differential braking may be produced.

One final thought. The method of reporting runway conditions in the United

States sometimes leaves a lot to be desired.

Meanwhle, Canada and certain European

countries have developed standardized

methodologies for measuring and reporting friction values and determining stopping distance, with excellent safety records.

iNe should follow their lead to help put an

end to unplanned runway excursions get

stop exactly where we intend. It makes for

a much nicer arrival. B&CA

www.AviationNow.comjBCA