13 Jul 2019 17:04

AUTHOR AvioConsult

Safety speed definitions and engine emergency procedures in pilot manuals of multi-engine airplanes do neither agree anymore with airplane design techniques and assumptions made during the design, nor with flight test techniques used to determine the safety speeds.

During the past 25 years more than 400 catastrophic accidents with multi-engine airplanes happened following the failure of one of the engines, during which over 4000 crew and passengers lost their lives. Most safety reports determined the cause of the accidents to be Loss of Control or Controlled Flight into Terrain, therewith more or less blaming the pilots, but the real cause is that most engine emergency procedures and clarifying definitions of minimum control and take-off safety speeds in pilot manuals do not include the strict manoeuvring limitations that result from the airplane design and certification requirements in CS/FAR 23 and 25. The mishap pilots are not to be blamed; the cause of such accidents is inappropriate engine emergency procedures and/or definitions of safety speed limitations in Airplane Flight Manuals and pilot learning books. A paper presented at the EuroControl Safety Forum 2019 discusses some of the errors and makes a plea for engaging high level multi-disciplinary knowledge in the development of safety-critical procedures.

The objective of the paper is to bridge the obviously existing gap between airplane design engineers and pilots, including procedure-writers, therewith improving procedures and safety, and reducing the number of future fatalities considerably.  Following a short video showing the loss of control of an airplane after engine failure at take-off, the paper briefly explains the CS/FAR approved weight- and cost-optimized vertical tail design criteria and discusses the consequences thereof for airplane manoeuvring and performance after engine failure.

Some basic knowledge of aerodynamics is included as required to understand aircraft motions after engine failure (at a popular science level) and an explanation of what happens immediately after engine failure, how to recover, maintain equilibrium and continue the flight safely as is customary during experimental flight testing. The effect of turns on controlability, performance and safety at low speed will also be discussed briefly.

Daily practice of procedure writers seems copying some of the certification criteria, published in CS/FAR 23.149 and 25.149, straight into airspeed definitions and procedures for use by pilots. These regulations however, are for the design and certification of airplanes, not for their operational use. Definitions and procedures definitely need to be amended for use by pilots. The real value of minimum control and take-off safety speeds are explained and an improved definition for pilot manuals is presented as well. An example of an improved engine failure procedure is included as well.

The emergency procedures for Air Traffic Controllers concerning engine failures at take-off might also require a review to be in accordance with the design criteria as an example shows.
Conclusion of this paper is that procedures for complex situations, such as maintaining control after engine failure, that are essential for the safety of flight, need to be well developed and approved by all disciplines that are engaged in the design, flight test and operational use of airplanes, which regrettably is not the case anymore today, given the deficient procedures and speed definitions of which examples are presented in this paper and the still large number of fatal accidents.

The video recording of the paper can be viewed at the EuroControl / SKYbrary website (under Technology and Science, 2/3 down the page, right hand side) here:

The pdf of the presented slides can be downloaded here:

The PowerPoint presentation including animations and some additional clarifying slides can be downloaded from the AvioConsult website:

The fully written 8-page paper can be downloaded  here:

On the website of AvioConsult,, several papers on the subject can be downloaded for free.

Author Harry Horlings, a graduate of the USAF Test Pilot School (1985), is available for presenting papers, contributing to multi-engine rated courses, reviewing training material, etc. Email address on website contact page.