Deceleration on the Runway
Deceleration on the Runway
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Description
The three primary methods of decelerating on the runway during the landing roll or a rejected takeoff are reversed engine power, brakes, and mechanical spoilers. Effective and coordinated use of whichever of these are available will result in a stopping distance appropriate to the available runway ahead, or the desired runway exit point if sooner. The key to success is the properly coordinated use of all available methods
Engine Power
The most important action to achieve deceleration from a speed at which the engines are still producing forward motion is to select all thrust/power levers to the ground idle position promptly, and if available, continue the action through to the selection of reverse thrust or reverse propeller pitch. This is the first action to begin deceleration and must especially be achieved without delay when a high-speed rejected takeoff is initiated. Thrust reversers and reverse propeller pitch are most effective at high speeds. Selection at these relatively high speeds must be symmetrical because otherwise, directional control may be prejudiced. Once the aircraft groundspeed has reduced sufficiently and stopping is assured, thrust/power levers should be returned to the ground idle position to prevent ingestion of any foreign object debris (FOD) which could be present.
For most transport aircraft, the effect of reverse thrust or reverse propeller pitch is not factored into landing performance calculations. Reverse thrust is simply taken as an additional safety margin.
Wheel Brakes
To be effective, braking action at any speed depends upon sufficient friction between the tyres and the runway surface achieved through freely rotating wheels.
Fundamental to this are:
- Achieving the maximum possible weight on braked landing gear assemblies
- The condition of the tyre tread
- The tyre inflation pressure
- The condition of the runway surface
Braking effectiveness will also be affected by the degree of brake wear, which must be within aircraft maintenance manual (AMM) limits, and the manually or automatically applied brake pressure as modified by system protections. These ensure adequate wheel rotation exists and is maintained before the commanded brake pressure is transmitted to the brake control units. When brake temperature indication is available on the flight deck, it must be within prescribed limits before a takeoff roll is commenced so that effective braking is available if a takeoff is rejected. System faults, short turn-around times after a previous landing with heavy braking, or inappropriate use of brakes during a long taxi can raise brake temperatures into cautionary ranges where a delay for takeoff may be required.
Anti-skid units are fitted to the braking systems of all modern transport aircraft. They modulate applied brake system hydraulic pressure before it is transmitted to the actuators in the brake units so as to obtain optimum braking. A minimum wheel rotational speed must be detected before any brake application will be achieved to prevent tyre destruction resulting from a locked wheel and to guard against the risk of aquaplaning on wet or icy runway surfaces.
Autobrake systems provide selectable rates of deceleration which usually vary between three and six knots per second constant deceleration rate. Selection of ‘Low’ autobrake on an aircraft equipped with thrust reversers will usually have the effect of delaying brake application to allow the thrust reversers to work efficiently in reducing the initial high ground speed. Maximum manual braking through the toe brakes can produce deceleration rates of up to ten knots per second subject to the operation of anti-skid units.
Modern landing gear assemblies on fixed wing transport aircraft are fitted with carbon brakes, although steel brakes may still be encountered on older aircraft. The application techniques for the two types differ slightly. Caution is required if a current aircraft type rating includes aircraft types or type variants which have both brake types. If this applies to either pilot, the subject should be included in pre-departure and approach briefings.
Although the validation of tyre tread and inflation are matters for Line Maintenance in accordance with the applicable Maintenance Programme, pilot preflight external checks should include a positive assessment of apparent brake assembly and wheel/tyre status, including brake wear indicators. Any resulting uncertainties should then be referred to maintenance personnel. It is not possible to reliably assess whether the inflation pressure of each tyre on a multi-wheel landing gear is within prescribed limits merely by visual inspection. The record of tyre inflation checks and restoration of prescribed minimum pressures should be available to operating flight crew by reference to the Aircraft Technical Log.
Mechanical Spoilers
The mechanical deflection of parts of the wing upper surfaces and tail cone assembly can assist deceleration in two ways:
- Directly, by increasing aerodynamic drag on the moving aircraft. This can be achieved by raising upper wing surface panels called ground spoilers or by operation of a tail cone ‘clamshell’ type air brake. Both systems can also be used in the air on some aircraft types as in-flight air brakes, but the extent of their operation may be less in the airborne case than in the ground (weight on wheels) case. Extreme care may be needed if it is permissible for a particular aircraft type to carry out a rejected landing after initial touchdown, since not all ground settings of deployed spoilers and air brakes necessarily auto retract to the settings needed for a safe initial climb away.
- Indirectly by increasing the effective downward load on the landing gear and thereby increasing the efficiency of wheel braking.
Appropriate Use of Deceleration Devices
Although runway lighting, marking, and signage may provide explicit indications of distance before the end of a runway, overrunning the end, either during a landing or a rejected takeoff, is not necessarily a consequence of systems failure. Rather, decisions about whether to maximise the use of deceleration systems are sometimes flawed because a poorly informed judgement is made about the ‘distance to go’. In the case of an abnormal landing roll or any rejected takeoff, the appropriate procedure is to maximise deceleration using whatever methods are available, taking account of the degree to which built-in system protections against inadequate wheel rotation are present.
Runway Surface Conditions
The effectiveness of deceleration from high speed on the runway after a landing or a rejected takeoff will be affected by the surface friction, with wet, slippery wet, or contaminated runway conditions posing additional challenges. The borderline between ‘wet’ and water contaminated (standing water) can be particularly difficult to determine. Flight crew often get little guidance from air traffic controllers, because controllers may not have water depth measurements and may only be permitted to offer 'unofficial observations' or to pass on pilot reports. Also, when snow or ice contamination exists, different types of friction measuring devices measure different friction values when used on the same surface. None of the friction measuring devices are reliable on all types of contaminations. This adds another level of uncertainty to the runway surface condition. Further guidance on runway surface conditions can be found in the article on the global reporting format (GRF).
Accidents and Incidents
Events where retardation methods were ineffective:
On 24 September 2022, a Boeing 737-400F night touchdown at Montpellier- Méditerranée was mismanaged after windshear associated with a nearby convective cell was encountered as the aircraft crossed the runway threshold. Without flight crew awareness, touchdown occurred well beyond the touchdown zone and the aircraft overran the runway and partially entered a shallow lake. The aircraft forward structure and both engines were extensively damaged, and the aircraft was declared a hull loss. The two pilots had changed roles at the captain’s suggestion soon after passing 600 feet aal but their subsequent cooperation was assessed as poor.
On 25 November 2022, a Boeing 737-800 landing at Kitchener/Waterloo Airport, Ontario, with the left thrust reverser inoperative overran the runway before stopping approximately 500 feet beyond the runway end. The captain had unintentionally selected the takeoff/go-around switch instead of disconnecting the autothrottles and then removed his hand from the left thrust lever to select reverse thrust on the right engine. The captain was experienced but new to type, and the first officer was a recently employed inexperienced pilot who had just completed his type rating. The left thrust reverser had been intermittently defective for the previous six months.
On 23 April 2019, the captain of an Embraer E170 which had just begun its standing takeoff at Yamagata, Japan, was unable to correct a deviation from the centreline and initiated a rejected takeoff at around 50 knots. This action was not immediately followed by maximum braking as required by company procedure for a rejected takeoff. The aircraft departed the side of the runway and continued on grass for almost 400 metres before stopping. The nose wheel steering system was found to be defective.
On 20 October 2023, a Boeing 737-800 made a normal touchdown at Leeds Bradford in strong crosswind conditions, but deceleration and an on-centreline trajectory was not maintained. It then veered off the left side of the wet runway at around 55 knots before quickly stopping in muddy ground six seconds later. The excursion was attributed to inappropriate management of both braking and directional control during the landing roll. To some extent, this may have been influenced by the sudden onset of nosewheel juddering due to wheel bearing failure, although this would have had no consequence for available directional control techniques.
On 12 March 2022, an ATR76-600 captain made an unstabilised approach to Jabalpur, India, leading to a first bounce more than halfway along the runway and a final touchdown 400 metres from the runway end. The first officer took control but did not commence a go-around, and the aircraft overran the runway before stopping. The captain had just over four months command experience and had made six similar "high-severity long-flare" approaches in the previous five days. These had gone undetected because although such exceedances were supposedly being tracked by company flight data monitoring, this event was not being tracked.
Further Reading
- ALAR Briefing Note 8.4 Braking Devices Flight Safety Foundation (2000)
- HindSight 12 - "Runway friction characteristics measurement and aircraft braking" by Werner Kleine-Beek
- An Investigation of the Influence of Aircraft Tire-Tread Wear on Wet-Runway Braking, T. Leland and G. Taylor, NASA, 1965
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