1.3.2. Controllability
NOTE: Yaw is the angle between the airplane centerline and a reference azimuth, i.e. yaw angle, can be read from change in the compass heading. Side slip is the angle between the airplane centerline and the relative wind.
For every condition of asymmetric thrust there is a minimum speed below which it is not possible to maintain equilibrium by aerodynamic controls. This speed will vary with thrust and fig. 1 illustrates the range of flight conditions under which it is possible to maintain equilibrium, that is, a condition in which the airplane is able to maintain a straight flight path.
If, the bank angle is zero (side slip is from the bad engine side) a large amount of rudder is required. The turn and bank indicator would show the ball in the center, since the wings are level and the flight path is straight. The deflected rudder generates a lateral force which pushes the airplane in a (steady) side slip.
Zero side slip is the case for least drag, and is the case for minimum exposure to possible roll-due-to-sideslip coupling problems. The aircraft is banked slightly towards good engine side. A lesser amount of rudder is being used than in case of zero bank, and the ball is slightly to the good engine. The lateral component of the wing lift force exactly balances lateral force from the rudder. The fuselage centerline is aligned with the flight path, hence no side slip exists.
A large bank angle is required with zero rudder deflection. The side slip is now from the good engine side. The rudder pedals would be neutral with the control wheel turned steeply towards the good engines and the ball far out to the good engine side. The fuselage side force generated by side slip is offset by the lateral component of the wing lift force, pointing in the opposite direction.
For each of the three situations illustrated, and for the all possible situations in between, there is a minimum speed at which unaccelerated, rectilinear flight can be maintained. This speed may vary very significantly as illustrated by the following example (DC-8-50, rudder power ON, flaps DOWN):
The certified Airborne Minimum Control Speed (VMCA) is equal to the lowest speed at which directional control can be recovered and maintained in flight under the following conditions:
Sudden failure of the most critical engine.
Take-off thrust on the remaining engine(s).
Flaps in take-off position.
Landing gear retracted.
Angle of bank not in excess of 5°.
During recovery, the heading change not to exceed 20°.
Up to 3° banking towards the operating engine(s) is normally used by the manufacturer in the certification of VMCA. This 5° bank lowers the VMCA as much as 30-35 kt for an aircraft with wing-mounted engines and in the order of 15 kt for aircraft with aft-mounted engines.
For aircraft with wing-mounted engines, a sudden engine failure may result in side slip angles of up to 15° in 5-6 seconds before the yawing can be stopped by rudder deflection, and the proper bank angle can be applied.
At large side slip angles, swept wing aircraft will develop large rolling moments. These rolling moments, counteracted by aileron and/or spoiler deflection, may require up to full deflection in a 15° side slip.
Controllability problems may be compounded in case of 2 asymmetric engines-out operation of a 4 engined aircraft. Training accidents in the past have learned that proper speed control during approach is extremely important. Too early speed bleed off below the (high) VMCA with two engines out increases the risk period unnecessarily.
1.3.3. Drag and Climb Performance
The control deflections to counteract yawing, side slip and roll and the increased drag of the less efficient wing in side slip considerably increase the drag of the aircraft.
Wind tunnel tests made on swept wing models have shown a drag increase of as much as 25% in a 12° trimmed side slip.
The least drag and hence the best climb performance is achieved at zero side slip which in practical terms means up to 3 degrees bank, depending on speed and the degree of asymmetric thrust.
1.3.4. Stall
In a side slip the wing has a reduced efficiency.
This is due to the trailing wing which will get an increase in sweep back resulting in increased spanwise flow of the boundary layer. This in turn will decrease the maximum lift of this wing.
On the leading wing, upward aileron and/or spoiler deflections reduce maximum lift.
Lateral spoiler deflection may also reduce the flap efficiency, thereby furthermore decreasing the maximum lift.
The above-mentioned reductions in maximum lift will increase the stalling speed by as much as 5% according to wind tunnel tests if side slip is not avoided by applying the proper bank angle.
1.3.5. Discussion
In case of engine failure at take-off or overshoot and heavy take-offs with an inoperative engine, the climb performance of the aircraft may be critical, at the same time the climb speeds used in these situations (V2) of a minimum margin above VMCA, may be close to the minimum control speeds (VMC) of the aircraft.
At high weights, V2 is normally governed by the required margin over the speed above VMCA (see fig. 1). Consequently the required bank is small or zero. This requires less manoeuvring drag and thus results in better aircraft performance than by using the more conventional bank. However, bank is needed to maintain the required track, especially at lower weights.
At low weights, V2 is frequently close to VMCA and thus more critical from a yawing point of view so that up to 5° bank may be required.
1.3.6. Summary
Asymmetric thrust conditions, occurring mainly during training of take-off and overshoots with wing-mounted engine(s) inoperative, will affect controllability, stall and climb performance.
In case of a sudden engine failure, up to 15° side slip may develop in 5-6 seconds, creating large rolling moments. Correction may require up to full aileron deflection. These control deflections may increase the drag by as much as 25% in a 12° steady side slip.
On swept wing aircraft a side slip may increase the stall speed by as much as 5%.
A bank of 3-5° towards the live engine(s) reduces side slip resulting in improved directional control and performance.
Up to an angle of bank of about 5° the VMCA decreases 4-8 kt/degree for aircraft with wing-mounted engines.
Excessive banking, however, will result in increased drag and can result in critical tail stall and loss of control.
At high weights V2 is normally well above VMCA and the side slip with wings level is small. At such weights, directional control can be maintained with wings level, simplifying the instrument flying procedure, thus giving a better average performance.
At low weights V2 is closer to VMCA and thus more critical from a directional control point of view. In this case up to 5° bank may be required to assist directional control.
In case of engine-out missed approaches immediate application of bank away from the failed engine may prevent serious controllability and other problems.
1.3.7. Crippled Aircraft
Mid-air collision, bomb explosion or other major structural malfunction may create a situation which is beyond the scope of the published abnormal or emergency procedures.
In such a case the crew may be required to perform (parts of) several different Emergency Checklist procedures.
The following guidelines are intended to assist the crew of a crippled aircraft:
Basic aerodynamics
- If aileron control is affected, rudder inputs can assist in countering unwanted roll tendencies. The reverse is also true.
- If both aileron and rudder control are affected, the use of asymmetrical thrust may aid roll and directional control.
- If elevator control is affected, stabilizer trim and thrust can be used to control pitch and aircraft attitude.
- Stall speed and VMCA increase with bank. The VMCA increase will be greater in a turn toward the dead engines than when turning the other way. Therefore limit bank to 15° if possible. With two engines on the same side inoperative (747) VMCA is significantly higher than with one engine inoperative.
- Aircraft can be landed using any flap position, including flaps up. This requires higher approach speeds and increased landing distances.
- In level flight the angle of attack has to increase as speed decreases. Lift increase on a damaged wing may less than on an undamaged wing, thereby creating a roll moment.
Flight path control
After a major failure the crew’s first objective should be to maintain or regain control of the aircraft and establish a safe flight path. This may require unusual techniques such as application of full aileron or rudder or even reducing power on operating engines. Fuel jettison (if available) should also be considered when aircraft performance appears to be critical.
Emergency containment
After a safe flight path is established perform appropriate abnormal/emergency procedures to contain the failure(s). Consider the consequences on other systems before performing an action (e.g. influence on flight controls when switching in hydraulic systems). If not absolutely necessary crew’s attention should be focused on containment of the problems and not on configuring the aircraft for immediate landing.
Damage assessment and aircraft handling evaluation
Unless circumstances dictate otherwise, the crew should take time to assess the effects of the damage.
If structural damage is suspected start with inspection of cockpit indications and try to assess damage by direct visual observation either from the cockpit or from any other accessible aircraft compartment.
If leading- or trailing-edge flaps damage is observed or suspected consider the possible effects on aircraft controllability when flaps are extended (asymmetry). Consider leaving the flaps up to the remainder of the flight, or only using the trailing-edge flaps if damage is suspected to the leading-edge flaps.
If controllability is in question, consider a check of aircraft handling characteristics and determine minimum safe speeds and appropriate configuration for landing. Slowly reduce speed avoiding large thrust changes and lower the flaps. Use the flap/speed schedule as dictated by the abnormal/emergency procedures.
If stick shaker or initial stall buffet are encountered or if full control column deflection or rudder pedal deflection are necessary to maintain an acceptable flight path, increase speed to a safe level and consider this speed to be the minimum approach speed for the established configuration. Consider that any further flap extension may worsen the asymmetrical condition. Any check of this nature should be made at an altitude that will provide a safe margin for recovery should aircraft handling be compromised.
If control is lost in a turn, returning to a symmetrical condition by reducing thrust may be the only way out. This will of course cost a lot of altitude. Lower the gear only if available thrust permits.
If aircraft performance is in doubt circumstances may make it necessary to do the configuration part of the check during the actual approach. Consider that alternate extension of gear and flaps may not be reversible.
Approach and landing
Large speed increments require excessive landing distance. Unusual aircraft configuration may create unusual aircraft attitudes during approach and landing. After landing use all available means to stop the aircraft. Circumstances will dictate whether the aircraft has to be evacuated on the runway or that taxiing clear of the runway is possible.