## Safet 

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# SlipSlidingAway

Know what to expect on arrival before you depart,

and then calculate accordingly.

ByPatrick R. Veillette,Ph.D. 

# ll e Citation 

### 500pilots were making a VOR approach to Runway 32at Sault Sainte Marie.After breaking out ofthe clouds at

### 2,500feet and aligning the airplane with the runway, the captain noticed "maybe compacted snow ormaybe icewith fresh snow" on the runway

- not surprising conditions in northern Michigan in February. When the aircraft was on atwo

- mile final, avoice came over the Unicorn frequency and said that braking action was nil. This observation gave further weight to

aNOTAM in effect atthe time that stated, "icyrunway, nil braking" atthe airport. The captain told the copilot they would perform

ago-around ifby midfield the aircraft was not decelerating adequately. The captain touched down within the first third ofthe runway asplanned, but as the Citation neared the midfield point, the airplane fishtailed. Passing midfield, the captain called for ago-around ashe added power and retracted the airbrakes. "There isnot enough runway!" the first officer called out and then braced. Moments later the twinjet ran offthe end ofthe 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 ago-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, ofcourse, the icyrunway. Surely those ofus who have been confronted with nearly identical conditions

- and that's probably quite afew

- 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. AFlight Safety Foundation study ofbus

### iness jet accidents between 1991 and 2003 found that 

### 59 ofthe 104landing accidents

www.AviationNow.comjBCA and all 176reported landing incidents involved overruns, with roughly 70percent of those involving skidding off the end of a contaminated runway. Aseparate analysis of FAA accident and incident data for years 1978to 1993uncovered 769instances ofaircraft overruns and excursions inwhich ice,

snow and slush were contributing factors. When reviewing weather reports for a destination, it'snatural toconcentrate onthe ceiling andvisibility to determine ifthe field is above minimums. But the runway's surface condition deserves close scrutiny as well. That said, the information is often unavailable

- especially ifthe facility isageneral aviation airport without an operating control tower. Even in the rare circumstances when braking reports are available, their reliability isoften questionable. Areport of "fair" braking from the pilot ofan aircraft with dual wheels all around, operable anti-skid and plenty oftire surface for added braking capability may not necessarily be representative ofthe braking condition for an aircraft with single-wheel gear whose anti-skid isinoperative ornonexistent.

Business & Connnercial Ariatio» • November 2005109Safety

Table 1 

### Airport Conditions Frequently Cited in Business Jet ASRSReports, 1991 to2003 Inadequate runway conditions, maintenance 33%

Poorfield maintenance (lights, surface condition, paint, markings) 28%

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

Lack ofweather reporting (no METARsand/or TAFs) 19%

Lack of adequate runway condition reports 18%

Lack of adequate snow removal 14%


### Note:Somereports containedmore thanonecategory.

Source: NASA ASRS

In fact, getting accurate information regarding the runway's surface is rather problematic. As part ofthe multi-year research behind the Flight Safety Foundation study, Ireviewed asample 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 ofafluid contaminant such aswater, slush, loose snow, deicing or hydraulic fluids or asolid 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 offluid 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 aswell. Of the 59business jet overrun accidents uncovered, 19involved rain-covered runways, as did 56ofthe 176incidents. Snow, slush and ice were factors in most ofthe 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 atleast the runway has aporous 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 setupon it. For example, aforecast ofmod110 Business & Commercial Aviation. November 2005erate or light rain on arunway with aPFC

or with grooves under most circumstances would normally bejust a"wet" runway. To the Europeans, at least, that means something quite specific. TheJAA defines a "wet" runway asone covered with water or having sufficient moisture to make the surface appear reflective, but without significant areas ofstanding water. PFC or grooved surfaces tend to hasten the drainage ofrain from the runway's surface and inhibit the build-up ofpuddles. During asummer deluge, these surfaces shed standing water in about 15minutes (that is, unless astrong 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 orheavier 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 JetAccidents 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. AirAccident "more than 25percent ofthe 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 asolid mass which resists further compression and willhold together or break into lumps ifpicked up; or byice, including wet ice." Bycontrast the FAAeschews such precision, and considers arunway contaminated "whenever standing water, ice, snow, slush, frost in any form, heavy rubber, or other substances are present." 

## -i.. Part 135requires operators to add 15per

## i ~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 awet runway increases the stopping distance by 25 percent to 100percent. According to the "Advisory Information" in the FAA approved flight manual for a Cessna Citation: Allflight manual field length data assumes adry, hard surface runway except where otherwise noted. Precipitation coveredrunway conditionswill degrade braking effectivenessand will require sif512ificantly greater actual takeoffabort 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: \-VetRunway -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 acontaminated runway as "a runwaywhere more than 25percent ofthe runway surface area, within the required length and width being used, iscovered by standing slush or snow more than 3 mm (0.125 in.) deep. Patches ofsnow, ice, and/or slush

on the takeoffllanding portion ofthe runway are also considered runway contaminates. Braking action, ifreported, isfair or good." In these conditions, the Falcon 900 AOM stipulates that afactor of 1.50should be added to the landing distance. Dassault defines aslippery runway asone
onwhich "standing water, snow, icepatches, and/or slush cover 50percent or more of the takeoffllanding portion ofthe runway. Braking action, ifreported, ispoor." In the

case ofaslippery runway, the Falcon 900 AOM stipulates that afactor 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 runway1.6to 1.7; Icy runway- 3.5 to 4.5. Bythe way, allofthese correction factors

assume the anti-skid system is fully functional. Obviously, operations on contaminated runways with either inoperative anti-skid orthrust reversers should beminimized unless absolutely necessary and even then only with allother factors within suitable safety margins. While these correction factors areuseful for determining the additional runway needed, it's important to note that they assume the runway composition, condition and construction, the amount ofprecipitation and the depth ofmain landing gear tire tread are allwithin satisfactory limits. And aswe allknow, 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. Ifrunway 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 pro

# Aerobraking
It looks great inthe T-38, but it doesn't work for businessjets

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 astop. 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, hetook control, but too late and the jet slid offthe end ofthe 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. Oneofthe problems noted inthe Flight Safety Foundation's recent business jet study was the frequency of overruns, a significant number ofwhich were attributed to incorrect braking techniques including aerobraking. Other studies have noted this problem aswell. 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 ofthe primary factors affecting braking in runway excursions or runway overruns has been a reliance onthe incorrect technique of maintaining anose-high attitude aftertouchdown 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 inthe North American T-38, long the U.S.Air Force's primaryjet trainer, orsimilar aircraft whose small thin wings were built for onething: speed. Thedisadvantage ofsuch designs was they required very high landing speeds. It's been over 20 years since Iflew the Talon, but Iseem 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 ourfeet off of the brake pedals above 100 knots lest we inadvertently blow atire. So how did we slow the T-38 upon touchdown? We held the nose ofthe aircraft upto 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 arange 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 ofthe 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 ofthe maintenance troops. Pilots in mufti who try to transfer this technique to the civilianworld fail to account forthe significant design differences between the average business jet and the supersonic Talon. These differences include aerodynamics, aircraft systems and momentum, all ofwhich dictate an entirely different method for dissipating the energy during landing. The wing surface
area alone ofthe average business jet is significantly grater than the T-38's. Moreover, the T-38 rolls onsmall 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. Oneofthe most overlooked and understated recommendations in the Flight Safety Foundation's ALAR manual is this potent statement: "The importance oftiming 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/Bcnducing minimum stopping distances; however, on aprecipitation-covered runway, the phenomenon ofhydroplaning may greatly reduce the anti-skid effectiveness, due tothe possibility ofthe airplane wheels not rotating up to a speed equal to the airplane's ground speed." Since hydroplaning may occur attouchdown, 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 astabilized approach with afinn touchdown in the touchdown zone to increase the probability ofwheel spin-up and to minimize the chances ofhydroplaning. The degree ofhydroplaning 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 onwet runways. Proper inflation is important because alow 

### tire pressure lowers the minimum hydroplaning speed. Anticipated operation on precipitation-covered runways dictates close monitoring oftire condition and pressure. Pilots should pay particular attention to adverse wind conditions when there is a possibility ofcontaminated runway conditions. The risk ofhydroplaning 

### is directly increased with atailwind 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 manyvariables, such astire tread design, runway material and surface condition, airtemperature and rolling speed ofthe tires. Oneofthe primary factors governing braking effectiveness is the amount ofweight 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 ofwhom also spent their formative years in the T-38) tried this rollout maneuver onjet transports, the added drag was not significant, andthe 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 ofthe aircraft's weight onthe main wheels, whereas keeping the nose inthe air puts negligible weight onthe main wheels. This fact is reiterated intraining literature published by Boeing, and earlier by McDonnell Douglas. Furthermore, some aircraft have nosswheel spin-up transducers which are required for optimum operation ofthe 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 onthe runway noticeably improves directional stability, especially on contaminated runways.) The task force also noted that extending ground spoilers results in85 percent ofthe aircraft's weight being placed onthe 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 assoon asthe 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 acarrier, 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 isthat afirm landing is better. Planting the gear ensures wheel spin-up forthe anti-skid system and in some aircraft for the extension of spoilers. Oncethe 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. Sofor maximum effectiveness, applythe brakes steadily to full pedal deflection assoon asthe ground spoilers are deployed and main wheel spin-up occurs. Thrust reversers should beapplied early at maximum power. Avoid using differential reverse thrust and ifone 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 beoperating before reverse thrust. The application of brakes tends to counteract any adverse pitch-up tendency that may develop. This isyet another reason reinforcing the importance ofthe timing of proper braking control inputs. Lastly, asageneral rule, it is recommended that braking continue until assured that the aircraft will stop within the remaining runway length. Manyof uswould loveto return to those earlyyears when we swagger outto the sleekT-38, don our "speedjeans" (a.k.a., gSUit),strap onand inandthen feelthe kick ofthose burners duringtakeoff. Butthosedaysarepastasaremanyofthetechniques that applied. Professional pilots are expected to usetile proceduresprescribedbythe manufacturer's AFM. Useofnon-standard techniques, especially during critical phasesofflight, can result in bent metal, andan assessment bytile FAAthat the pilot

- a rightstuff resumenotwithstanding

- hadbeen"careless andreckless" andwrong. 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 acontaminated 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 icyrunway to amere 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

### 25percent to the runway requirement. Pilots should

usemaximum reverse thrust assoon aspossible after touchdown since its effectiveness isgreater athigher speeds. Thrust reservers willdecrease the stopping distance on aslick runway by 10to 30percent. According to Boeing research, excessive cycling ofan anti-skid system reduces total braking effort. Maximum braking occurs when only enough brake pedal pressure isused to produce an occasional anti-skid release, or about one release over two to three seconds. Youcan feel this antiskid cycling asa"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 aturning/cornering force, and the reduced friction on a contaminated runway compromises the aircraft's directional control. Maintain control through the rudder pedals. Use ofnosewheel steering above taxiing speed may cause the nosewheel to hydroplane and result in the loss ofnosewheel cornering force, with aconsequent loss ofdirectional control. When differential braking is necessary, apply pressure on the required side and but not on the opposite side to regain directional control. Ifbraking isnot 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 alot 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

amuch nicer arrival. B&CA

www.AviationNow.comjBCA