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Controlling airplanes after engine failure

Report Airplane Control after Engine Failure

Paper The Effect of Bank Angle and Weight on V_MCA

Paper Staying Alive with a Dead Engine

Report Imperfections and Deficiencies in FAR and CS

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Prevention of Airplane Accidents after Engine Failure

Introduction.  Five years after the beginning of aviation, Orville Wright and passenger Lt. Thomas Selfridge took off to conduct a test flight in a new aircraft.  In a tight turn, at a height of approximately 150 ft, one of the propellers broke and Orville lost control; the plane plunged to the ground.  Lt Selfridge was killed and Orville Wright himself was seriously injured.  Aviation claimed its first victim, the cause being the loss of control due to asymmetrical thrust.  The Wright bothers' aircraft had one engine, driving two propellers and were therefore to be considered multi-engine airplanes.  Orville did not know yet how to control an airplane after the failure of one of the propellers. 

Today, 100 years later, engine failures of multi-engine airplanes continue to take their toll.  All across the globe, accidents with both small and big multi-engine airplanes continue to happen quite frequently following the failure of an engine during takeoff, go-around, approach for landing and during training, despite the fact that all airplane types are thoroughly flight-tested and operational limitations are published in the airplane flight manuals.  The question how accidents after engine failure can be prevented is therefore actual since the beginning of aviation. 
Below, this question will be answered.  A rowboat will serve as an example to begin with.

Similarity between the controllability of a rowboat and an airplane after 'engine' failure

If you are in a rowboat and one of the side-by-side sitting oarsmen quits rowing while the remaining oarsmen continue at maximum effort, the propulsion has become asymmetrical causing the rowboat to start turning (yawing).  The helmsman will deflect the rudder to avoid the yawing, but the rudder is effective only down to the speed at which the maximum rudder deflection is reached.  This speed is called the minimum control speed (Vmc) of this rowboat.  

Changing the longitudinal center of gravity affects the rudder generated yawing moment (N_dr) as well, because the moment arm, the distance from cg to rudder, changes.  Moving the center of gravity aft decreases the rudder moment arm, making the rudder les effective, so a higher speed is required to maintain a straight course; Vmc becomes higher.  An aft cg is therefore the worst case.
While the boat is tipping over a few degrees away from the failed oarsman, a side component of the weight develops in a direction opposite of the rudder generated side force, reducing the sideslip (drift) and therewith the drag to a minimum.  For multi-engine airplanes, this similar characteristic is used for designing and sizing the vertical tail. 

The size of the rudder, the magnitude of the thrust asymmetry, the tipping angle and the location of the center of gravity play an important role for the magnitude of Vmc of a rowboat.  On a multi-engine airplane this is not different, although the loss of directional control leads to an uncontrolled descend, while a boat remains afloat.  Tipping over into the good engine in an airplane can be easily achieved by banking using the ailerons, rather than shifting weight laterally, like in a rowboat; airplane passengers have their seatbelts fastened.  Hence, the bank angle is an important variable as well.  This will be further explained in the next paper.  

It is recommended to read this 3-page paper first:

Controlling Airplanes after Engine Failure

Tail Design Imposed Limitations

Airplanes are designed, flight tested and certified to continue to fly safely after engine failure or while an engine is inoperative.  After reviewing many accident investigation reports using the knowledge gained at the USAF Test Pilot School, it was noticed that most pilots and accident investigators explain and use the minimum control speed V_MC in a different way than airplane tail design engineers, experimental test pilots and flight test engineers do. This difference in interpretation has, to the opinion of AvioConsult, often led to catastrophic accidents caused by the loss of control and/or performance after engine failure and also to incorrect and incomplete conclusions and recommendations in accident investigation reports.

The objective of this paper is to prevent accidents following the failure of an engine.  To achieve this, a few aspects of the design of the vertical tail of a multi-engine airplane and of the flight test techniques that are used to determine the minimum control speed V_MCA (Vmc in the Air, or Airborne) are discussed for the readers to become aware of the real value of V_MCA that is listed in Airplane Flight Manuals (AFM) and to learn about the conditions for which V_MCA is valid and about the V_MCA related deficiencies that exist in AFM's, training manuals and engine emergency procedures.

Conclusions of the paper: The vertical tail of multi-engine airplanes is designed and sized only for maintaining straight flight after engine failure down to the AFM listed minimum control speed V_MCA, while maintaining a bank angle of 3 - 5 degrees (as opted by the manufacturer for sizing the vertical tail) away from the inoperative engine and while the power setting of the opposite engine is max. takeoff.  However, this essential and life-saving condition for the AFM listed V_MCA to be valid is regrettably not included anymore in the operational limitations and engine emergency procedures of most multi-engine airplanes and not in training books either.  In addition, the definitions of V_MCA in AFM's and training manuals are usually not i.a.w. the tail design criteria and V_MCA flight test techniques. Pilots, unaware of these conditions, perform maneuvers, while the airspeed is at or above the AFM-listed V_MCA when an engine is inoperative, that the airplane was not designed to do, which has led and will lead again to catastrophic accidents, unless improvements are made.  
While maintaining the small 3 to 5 degree bank angle away from the inoperative engine (not just max. 5 degrees without specifying the direction, as listed in most flight manuals), not only the actual V_MCA is lowest, but also the sideslip angle is smallest, so the drag is lowest possible, leaving maximum available climb performance while an engine is inoperative. Refer to the report presented below for many suggestions to improve airplane control after engine failure, including regulations, manuals, procedures and training for flight with an inoperative engine.

The Dutch magazine Piloot en Vliegtuig (Pilot and Airplane) published this article in their December 2008 issue.

VR and V2MIN.  The standardized, AFM-listed VMCA is one of the factors for calculating the rotation speed VR and the minimum takeoff safety speed V2MIN of big Part 25 airplanes.  Since this VMCA is valid only while maintaining a bank angle of 3 to 5 degrees, as to be specified by the manufacturer, away from the inoperative engine, both the calculated VR and V2MIN are also valid only when maintaining the same bank angle (when the thrust setting is maximum takeoff).

This figure, a safety improving suggestion out of the report presented below, shows that the actual VMCA in this example has become higher than VR because the wings are kept level.  Bank angle and rudder advisories are presented to decrease the actual VMCA to a safe level to prevent the loss of airplane control.  The bank angle advisory widens up as airspeed increases.

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