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Coping with emergencies

Overcoming aircraft control system failures

by Phil Stonebanks


Rev. 1 — page content last changed
July, 2004
  
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One of my early instructors was a highly pessimistic individual, always muttering about 'What if this bit fell off, how would you cope with it?" and other such comments full of joy. Over the years however, I have come across a number of incidents where things have fallen off, with widely differing results.

A few years ago a gliding friend suffered a failure of an aileron quick-release control rod which caused the free aileron to flutter. An uncomfortable but still somewhat controllable situation. Unfortunately when she tried to turn the glider the loose end of the rod jammed in the structure and resulted in a high speed fatal descent. Another friend found himself at the top of a glider winch launch with no elevator control and escaped by parachute from only 600 feet.

A third gliding friend flying a Nimbus 3 (an 87 feet span monster with several flap/aileron sections) aero tow-launched after a servicing during which the controls 'adjusted'. The test flight revealed the glider would only circle to the left despite full right aileron and rudder. After an interesting launch, where the tug pilot managed to turn and climb at the best rate for the glider, he was dropped off tow high enough to bale out. By experimenting with various speeds, flap and airbrake settings he managed to control it just enough to return to the airfield. By having a sound knowledge of the aircraft and approaching the problem in a calm efficient manner this pilot recovered a situation which might have ended very differently.

The aim of this article is to encourage pilots to think about how they might cope with a control problem, and what aspects of the aircraft behaviour might be of assistance. While most control failure problems can be avoided by suitable maintenance there is always the possibility that one day you may find yourself with such a challenge. The key to surviving such an experience is a thorough knowledge of the handling characteristics of the aircraft, particularly the secondary effects of controls.
Control failure modes
Control failures, be they caused by mechanical failure, collision or structural failure will probably result in one or more of the following:
  • restricted or no movement of the control surface
  • surface floating free and probably fluttering to some extent
  • surface missing completely, or connected by control cables and probably flailing around behind the aircraft
  • major application of one or more controls to remain in a desired attitude/heading.
Pitch control
Perhaps the most critical control and also the one with most options, as most aircraft can be controlled in pitch in a number of ways.

Adjusting the power setting will usually result in a trim change. Coarse or gentle applications of power may have different effects on attitude, descent rate and the all-important airspeed. Varying the power will also adjust torque and slipstream effect, thus assisting, to a small degree, with roll control.

Aerodynamic trim tabs (those that sit on the trailing edge of the elevator) may be of some use. If the surface is jammed the tab will work like a small, albeit not very effective, elevator, although the lever must be moved the opposite way to the control column. (Trim lever forward will raise the nose). If the elevator is floating free (and not fluttering) the trim lever may be used in the same sense as the control column.

Bank angle is an effective way of controlling pitch. We all know that as we enter a turn the nose tends to drop unless we counter it. It is possible to use bank angle to lower the nose and hence control the speed. You will of course be in some sort of (probably descending) turn but the turn will be partially controlled and that is better than spinning or stalling. The steeper the bank, the more the nose pitches down. By adding pro-turn or anti-turn rudder more control is available. This method gives you a reasonable degree of control over speed, in return for some height loss and the increased risk of a cross-controlled stall.

Centre of gravity. If your aircraft has more than one tank and you can transfer fuel you may be able to adjust the attitude by moving fuel. Even leaning forward or back will have some effect. It's not much but it's better than nothing. A Miles Messenger escaping to England during WW2 lost its entire engine after one propeller blade was shot away while crossing the Channel. The family aboard all piled into the front seats and the aircraft glided just above the stall to shore and a successful landing!

Several aircraft have approach control devices such as flaps or spoilers. These controls usually have some sort of trim change associated with their operation. Some higher performance gliders use flap settings even more than the elevator for controlling pitch, relegating the elevator to little more than a trimming device for much of the time.
Roll control
In the event of loss of the ailerons some degree of roll control is available by using the secondary effect of rudder. While not an efficient way to turn the aircraft you should have at least some directional control.

Short or rapid bursts of power may increase the effectiveness of the rudder to some degree. Power, in the form of torque and slipstream effect may also be of use.
Yaw control
Loss of the rudder, as long as the aircraft is kept away from a stall/spin poses the fewest problems as long as the effects of power and adverse yaw are understood. Bank angle can be used to counter any yaw tendency (from torque or a damaged fin while in flight) and care must be taken to allow for adverse yaw when entering or exiting any turns.
Effect of airspeed
The various trim changes associated with controlling the aircraft change with the airspeed. Adverse yaw for example, decreases with an increase in airspeed. The aircraft should be flown at a speed safely above the stall but no faster, unless the increase in speed provides more control. If a control surface is floating free it will tend to flutter and the violence of the flutter will increase with speed.
Other methods
Some aircraft have doors or canopies fitted which if opened in flight may well provide some sort of trim change. It may or may not be of use but it is worth considering. If the aircraft is approved, the manufacturer may be able to supply information on what happens when a door is opened in flight. Unless the door is approved for opening in flight it should not be practised, but in an emergency ...
Control on the ground
If possible try to find somewhere to land which is large, long and flat, and as into wind as possible. On the ground the ailerons (or more accurately, adverse yaw) can be used to aid in directional control. (Use left aileron to turn right).

Those with differential brakes can make use of them for some directional control. As many of us fly taildraggers with single brake controls, use brakes only gently and while travelling straight. Heavy braking when the aircraft is starting to swing will accelerate the impending ground loop.
Considerations
When faced with some control problem you should endeavour to place the aircraft in a reasonable attitude with sufficient speed for normal flight or as near as possible to it. Assess the failure as to what type (whether the failure is a structural or mechanical one and whether the surface is still there, fluttering etc.), which control(s) are affected and the various secondary effects that can be used to help.

In the event of a structural failure or collision the airframe integrity will already have been compromised and so the aircraft should be landed as soon as possible, once some manner of control has been established. Extending flaps or spoilers, or opening doors on a damaged airframe may compromise the structure further so, unless control is inadequate, leave flaps where they are.

If the control surface or the structure is fluttering, once again, land as soon as control is established. Flutter can be very violent and destructive, and will increase dramatically with airspeed, so aim to fly at the minimum speed where you still retain sufficient control.

If the control failure is a restriction or loss of movement and you have the aircraft under sufficient control, it may be flown to a more suitable area for landing. However, continue to monitor the problem and be prepared to land immediately if there is any sign of the problem compounding.

All of the above methods of alternative control, even those that only provide a small measure of assistance, may well add up to the difference between surviving a control problem or not. Even if you never have a control failure, considering the above methods will hopefully make you more aware of the aircraft's habits and so improve your flying skills.

Many of these methods may be practised safely at a suitable height. For example, trim the aircraft to fly "hands off' and try a number of climbing and descending turns, rolling out onto specific headings, using rudder and effects of power. With an aircraft as responsive to secondary effects as a Drifter it is possible to fly entire circuits without touching the stick, but it is best to practise this with an instructor.

Hopefully none of us will ever have to cope with a control failure but it would be nice to know how the aircraft (and the pilot) might react to one. After all none of us want to have an engine failure but we all practise in case we get one (don't we ... ?).

Read the article 'Rooted' in the May – June 2004 issue of CASA's Flight Safety Australia magazine.



The next section of the Coping with Emergencies Guide is a section of the 'Flight Planning and Navigation Guide' that deals with the 'procedure when lost'

Groundschool – Coping with emergencies

| Guide contents | Knowing the aircraft | Deceleration forces | Forced landing procedures |

| Overcoming aircraft control failures | Procedure when lost | Safety and emergency communication procedures |

| Aviation distress beacons | Understanding SAR services | ERSA emergency and survival procedures |



Copyright © 1996 Phil Stonebanks