By: Mark H Goodrich – Copyright © 2001
[This technical bulletin was prepared for flight test and ferry crew personnel with the International Ferry & Flight Test Group at Aviation Consulting Enterprises Co., and reflects flight standards and procedures for that organization.]
Experience reflects that many flight crew personnel have misconceptions about the most effective methods of reducing airplane speed during landing or an aborted takeoff. In addition, some have developed bad habits in line operations, where long runways provide little incentive to hone skills and use procedures required to minimize stopping distances. The following issues are each relevant to the process, and are presented below in their descending order of importance.
Airplane braking is the single most important factor in stopping an airplane. Although there are many collateral issues (e.g., anti-skid system performance, airplane weight, tire condition and inflation, type of brake system installed, condition of the brake system components, and coefficient of friction available due to the temperature and condition of the runway surface, contamination or clutter) that affect the overall braking performance, wheel braking is the key to stopping performance in substantially all cases.
When landing, the most effective braking technique is to have the brakes engage as the main gear wheels spin up, and continue braking through taxi speed. If runway length is limiting, maximum braking – that is, to anti-skid system release values – at higher speeds is the best procedure, in order to slow the airplane to the maximum extent possible before brake, wheel and tire temperatures increase. While carbon brakes continue to provide braking performance at high temperatures, increased wheel and tire temperatures can cause fuse plug release, which does dramatically decrease braking capacity, and can also result in fire, and in damage to the tires, wheels, gear assembly components, and airframe or other systems.
If runway length is not limiting and with auto-brake systems, selection of Autobrake 1/Autobrake Minimum is generally the best procedure with steel brakes, as it results in braking initiation during wheel spin-up, but maintains a low-level brake energy profile with attendant lower temperatures. With carbon brakes, extending brake life requires that the brakes be heated up quickly, and application events minimized. Therefore, selection of Autobrake 2/Autobrake Medium is the best procedure with carbon brakes, as it results in braking initiation during wheel spin-up, but with a high enough deceleration setting to prevent repeated brake-release events during the process. Taking over manual braking by pressing the rudder pedal brakes to over-ride and release auto-brakes will allow for a single “application event” from airplane touchdown through taxi speed. If runway length or runway surface condition is limiting, selection of more aggressive auto-brake settings, or using more aggressive manual brake applications early in the rollout sequence is most effective.
Control of the final approach and touchdown speed is the second most critical element to stopping performance. Energy increases exponentially with a ground-speed increase. Use of maximum landing flap and minimum target speed additives for the headwind component and gust values are the keys to minimizing energy at touchdown.
Contrary to the recommendations or requirements at many airlines, a managed or automatic speed calculation by the FMS is best discontinued not later than the Final Approach Fix inbound, and replaced with a manually selected command target speed, where runway length or braking conditions are critical factors. This prevents the addition of excess speed by conservative FMS programming algorithms, which can easily result in an additional 10 to 15 knots over target speed at the runway threshold if tailwinds or other factors are sensed on final approach.
Touchdown Zone Control
Landing within the touchdown zone is a function of controlling speed and altitude across the runway threshold, and understanding that quality of the touchdown is far less important than being on the runway, with ground spoilers extended and wheel braking underway. The airplane does not decelerate meaningfully while flying in ground effect, but does once on the runway with ground spoilers extended. Indeed, for many airplane types, the airplane actually accelerates in ground effect due to the inherent reduction in induced drag values. When runway length and condition are not limiting, the normal touchdown point is .25 nm/1500 feet/500 meters down the runway, and the normal touchdown zone ranges from 1,000′ to 2000′ from the threshold. Under normal circumstances to a long runway with ILS guidance, the threshold crossing height for a stabilized approach to the normal touchdown point is +/- 60 feet. When runway length and condition are limiting, and when the airplane can be flown from not lower than 600 feet HAT under VMC, the minimum threshold crossing height may be reduced to not lower than 20 feet HAT.
Landing at the touchdown point requires minimum target speeds, such that some energy is lost during the flare, following which the airplane is “allowed” to touch down immediately, rather than being held in the flare while speed dissipates slowly.
Ground Spoiler Deployment
Braking effectiveness is substantially enhanced when effective weight on the main gear is increased. Because the primary airfoil produces a considerable lifting force even at relatively slow airspeeds, deployment of the ground spoilers is an important factor in the process of creating a high coefficient of friction between the airplane tires and the runway surface.
Although most modern turbojet airplanes are equipped with auto-spoiler deployment systems, some older models (e.g., early model DC-9, B727 and B737 airplanes) have no automatic deployment system for ground spoilers. More importantly, the failure of an auto-spoiler system to work – either because of a mechanical defect or a failure by the flight crew to arm the system for automatic operation – can create very significant reductions in braking effectiveness. It is the responsibility of the PF to ensure that the ground spoilers deploy either automatically or manually, and that of the PNF to back up the PF by immediately announcing any failure of the ground spoilers to deploy.
A frequent procedural error observed in the training of flight crew personnel is a “rush” to deploy spoilers, with the result that they are deployed while still in the air or with a delay in main wheel braking while the PF fumbles around to grab and pull the ground spoiler deployment lever. Deployment while in the air can impose enormous forces on the landing gear, engine pylons, wing, horizontal stabilizer, and attached components. A delay in braking is far more significant to extending the stopping distance than a delay in spoiler deployment, and the best procedure is to ensure that braking is underway, and then purposefully reach for the lever and extend the spoilers smoothly. In addition, many airplane types (e.g., DC-10, L-1011, MD-11) experience a noticeable “pitch up” when the spoilers are deployed, and steady deployment is thus also related to the maintenance of airplane control.
Although not addressed by most military and airline operations, aerodynamic braking is the next most important component to stopping the airplane. Many line pilots have developed the habit of pushing the control column forward after touchdown. In conjunction with braking, this procedure moves effective weight forward to the nose gear (where there are no brakes), lightens the effective weight on the main gear (where there are brakes), renders the airplane laterally and directionally unstable, reduces drag exposure of the primary airfoil to the passing air stream, and increases loading forces on the nose gear to unnecessarily high levels.
The most effective way to use aerodynamic braking is to smoothly but affirmatively “fly” the nose down to the runway after main gear touchdown, and then progressively pull back on the elevator control after spoiler extension is complete and main wheel braking has been initiated. The most effective amount of aft elevator is the maximum that can be applied without raising the nose tires from solid contact with the runway. This procedure aerodynamically shifts the effective weight of the airplane aft from the nose gear to the main gear, and substantially increases the coefficient of friction between the main gear tires and the runway, thus increasing braking effectiveness. Continued use of aileron to counteract crosswind forces and keep the wings level during the roll-out must be maintained – and is critical on some types (e.g., A-340, B-707/720, B-747, DC-8) – with greater amounts of aileron displacement required as speed decreases.
Most pilots rely too much on thrust reversers as a stopping mechanism. While the older “clam shell” type reverser systems provided effective reversing forces even at lower speeds, the “blocker door diversion” systems used with high-bypass engine types provide substantial net reversing forces only at higher airspeeds – that is, at grounds speeds above 120 knots. Normal touchdown speeds – even with wide-body types at lighter landing weights – are often at or below the range of reverser effectiveness. Remember that, with high-bypass engines, the fan bypass air is being diverted laterally with some reverse thrust component, but the engine core is still producing forward thrust. Therefore, the net reverse thrust component is substantially less than might be indicated by the engine noise and vibration. When considering operational effectiveness, the increased susceptibility to foreign-object damage, and the affects to engine wear and maintenance, the use of idle reverse is suggested for the landing roll out down to 40 knots or so, while reverse thrust above idle is suggested for use at ground speeds only above 120 knots.
As with the discussion above regarding ground spoiler deployment, flight crew personnel are often observed in a “rush” to quickly deploy the thrust reversers as the airplane touches down, resulting in a sometimes frantic pull against the interlock system because it has not had time to release before the reverser levers are pulled up. This process often results in a delay of both braking and spoiler deployment, and sometimes takes attention away from fundamental airplane control, as well. The important point is that reverser deployment is far less important to stopping the airplane than are braking and ground spoiler deployment. Once the nose gear has been flown to the ground, braking initiated, ground spoilers smoothly deployed and aerodynamic braking initiated, the interlock mechanisms will have had time to engage, and a smooth effort to select the idle reverse position can be undertaken.
Additional Considerations – Pavement Condition
At airports with deteriorated asphalt runways, pieces of pavement can be easily ingested into modern jet engines if high thrust settings are used at slow speeds. For that reason, high thrust settings should be delayed when taking off from such surfaces until at least 50 knots ground speed. When landing on such asphalt, thrust above idle reverse should be restricted to use only at speeds above 120 knots ground speed. In addition, aggressive braking on landing can break pieces of asphalt loose, after which they can be flipped up from the aft main gear tires and into the trailing edge flaps, lower wing surface, main gear wells, or aft-mounted engines. On such surfaces, speed control, early touchdown, aerodynamic braking and light to moderate braking from main gear wheel spin-up are therefore critical if aggressive wheel braking is to be avoided during the landing roll.
Additional Considerations – Tire and Brake Condition
In the case of tires or brakes that are significantly worn, or in the case of brakes that have been partially deactivated, it is critical that the crew analyze tire and brake condition during the preflight inspection, and factor their status into the planning for a potential rejected takeoff, and for landing.
Additional Considerations – Wet Runways
A wet runway surface results in a reduced coefficient of friction between the tires and runway surface, extending the distance required to stop. Not only are coefficients of friction reduced by a wet surface, but the airplane may “hydroplane”, either on standing precipitation or steam that results from the energy at the tire-runway interface. Aircraft weighing more than 300,000 pounds quite easily create steam with resulting and associated rubber-reversion hydroplaning under normal braking on a wet runway, and the more intense braking during a rejected takeoff results in or exacerbates the situation for even smaller airplane types.
Reverse thrust can be quite helpful in the high-speed regime, assisting in deceleration through the most critical speeds for hydroplaning without heavy braking. However, in high crosswind conditions, the use of reverse thrust on landing or takeoff rejection produces a negative thrust vector with the same effect as locking the front wheels on a car, and then trying to steer on a slippery surface. Reverse thrust may have to be discontinued and a positive thrust vector reestablished to regain directional control.