Chapter 15-Transition to Jet Powered Airplanes
Table of Contents
Jet Engine Basics
Operating the Jet Engine
Jet Engine Ignition
Thrust to Thrust Lever Relationship
Variation of Thrust with RPM
Slow Acceleration of the Jet Engine
Jet Engine Efficiency
Absence of Propeller Effect
Absence of Propeller Slipstream
Absence of Propeller Drag
Recovery from Overspeed Conditions
Mach Buffet Boundaries
Low Speed Flight
Pilot Sensations in Jet Flying
Jet Airplane Takeoff and Climb
Rotation and Lift-Off
Jet Airplane Approach and Landing
The Stabilized Approach
Touchdown and Rollout
TOUCHDOWN AND ROLLOUT
A proper approach and flare positions the airplane to touch down in the touchdown target zone, which is usually about 1,000 feet beyond the runway threshold. Once the main wheels have contacted the runway, the pilot must maintain directional control and initiate the stopping process. The stop must be made on the runway that remains in front of the airplane. The runway distance available to stop is longest if the touchdown was on target. The energy to be dissipated is least if there is no excess speed. The stop that begins with a touchdown that is on the numbers will be the easiest stop to make for any set of conditions.
At the point of touchdown, the airplane represents a very large mass that is moving at a relatively high speed. The large total energy must be dissipated by the brakes, the aerodynamic drag, and the thrust reversers. The nosewheel should be flown onto the ground immediately after touchdown because a jet airplane decelerates poorly when held in a nose-high attitude. Placing the nosewheel tire(s) on the ground will assist in maintaining directional control. Also, lowering the nose gear decreases the wing angle of attack, decreasing the lift, placing more load onto the tires, thereby increasing tire-to-ground friction. Landing distance charts for jet airplanes assume that the nosewheel is lowered onto the runway within 4 seconds of touchdown.
There are only three forces available for stopping the airplane. They are wheel braking, reverse thrust, and aerodynamic braking. Of the three, the brakes are most effective and therefore the most important stopping force for most landings. When the runway is very slippery, reverse thrust and drag may be the dominant forces. Both reverse thrust and aerodynamic drag are most effective at high speeds. Neither is affected by runway surface condition. Brakes, on the other hand, are most effective at low speed. The landing rollout distance will depend on the touchdown speed and what forces are applied and when they are applied. The pilot controls the what and when factors, but the maximum braking force may be limited by tire-to-ground friction.
The pilot should begin braking as soon after touchdown and wheel spin-up as possible, and to smoothly continue the braking until stopped or a safe taxi speed is reached. However, caution should be used if the airplane is not equipped with a functioning anti-skid system. In such a case, heavy braking can cause the wheels to lock and the tires to skid.
Both directional control and braking utilize tire ground friction. They share the maximum friction force the tires can provide. Increasing either will subtract from the other. Understanding tire ground friction, how runway contamination affects it, and how to use the friction available to maximum advantage is important to a jet pilot.
Spoilers should be deployed immediately after touchdown because they are most effective at high speed. Timely deployment of spoilers will increase drag by 50 to 60 percent, but more importantly, they spoil much of the lift the wing is creating, thereby causing more of the weight of the airplane to be loaded onto the wheels. The spoilers increase wheel loading by as much as 200 percent in the landing flap configuration. This increases the tire ground friction force making the maximum tire braking and cornering forces available.
Like spoilers, thrust reversers are most effective at high speeds and should be deployed quickly after touchdown. However, the pilot should not command significant reverse thrust until the nosewheel is on the ground. Otherwise, the reversers might deploy asymmetrically resulting in an uncontrollable yaw towards the side on which the most reverse thrust is being developed, in which case the pilot will need whatever nosewheel steering is available to maintain directional control.15-24