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Products page contents: Report Airplane Control after Engine Failure Paper The Effect of Bank Angle and Weight on VMCA Paper Staying Alive with a Dead Engine |
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Flight crew procedures. Because of these design considerations (hardware) and, hence, inherent limitations of the vertical tail for the lateral control of the airplane, it is required to design operational procedures (software) to ensure that an engine failure and the limited size of tail and rudder do not lead to the loss of control and to catastrophic accidents. For this purpose, the airplane is flight-tested during which the lowest airspeed is determined at which straight flight can be maintained with the most critical engine made inoperative. During the test, most variables that have influence on the engine-out equilibrium of forces and moments are kept at their worst case value, returning the highest -- the worst case -- speed for maintaining lateral control. This speed is called the minimum control speed in the air, VMCA. The bank angle used during measuring VMCA is the same as was used to design the vertical tail: mostly 5 degrees away from the inoperative engine, but it is up to the manufacturer to select an angle, provided it is max. 5 degrees. VMCA is published as a limitation in the Flight Manual of all multi-engine airplanes. The smaller (and cheaper) the tail, the higher VMCA would be, but Regulations require the VMCA to be less than 1.2 VS. This is therefore a factor that links the hard- and software. Another factor for the tail design is that VMCA should be as low as possible since the lift-off speed is then lower, enabling shorter runways or higher payload. Refer to the Regulations and /or to the report presented below for other limits to control deflections and forces.
Many
pilots consider VMCA a useless speed, but it is still determined and
indicated on the airspeed indicator of Part 23 airplanes by a red radial
line – for a purpose, which is the limited size of the
vertical tail!
On big Part 25 airplanes, rotation speed VR and takeoff safety speed
V2
are used instead of VMCA, but VMCA is used to calculate these
procedural speeds.
VMCA and/or V2
are determined to be used as operational limitations by all pilots to ensure a safe takeoff,
go-around and flight, whether or not an engine fails or is inoperative.
Despite these safety ensuring speeds, catastrophic accidents continue to happen,
both after engine failure and during flight while an engine is
inoperative.
After reading many
accident investigation reports, airplane flight manuals and student pilot
textbooks, AvioConsult noticed that (airline) pilots have a very
different understanding of VMCA than is intended by regulations, is used by manufacturers for designing the vertical
tail and is used by experimental test pilots during flight-tests to determine VMCA.
The mishap pilots did not maintain straight flight and did not maintain a
small bank angle away from the inoperative engine while the airspeed was
low and the (asymmetrical) engine thrust was high. The
aerodynamic force generated by tail and rudder became too small to counter the
asymmetrical engine thrust.
VMCA is not just a constant. Following additional research, it became clear that VMCA is not just a constant number, but that VMCA varies a lot with bank angle. If the wings are kept level instead of 5 degrees away from the inoperative engine, VMCA is at least 8 knots higher than the VMCA that is listed in the Airplane Flight Manual (AFM). For Boeing 707 and DC-8 type airplanes, the actual wings level VMCA is 35 knots higher than the AFM listed VMCA that was determined while banking 5 degrees away from the inoperative engine!
An
example: In the linitation section of the AFM of a multi-engine
airplane, a VMCA of 95 kt is listed. This VMCA was
determined while banking 3 degrees away from the inoperative engine, which
was in this example the left engine, and at low gross weight (red line).
The red line (dip at 5 degrees) presents the actual VMCA while
the airplane is at low weight (the test weight); the blue line shows the
actual VMCA at high weight, both while varying the bank angle and
with max. thrust set on the opposite operating engine.
The figure, that is
calculated using the stability derivatives of the airplane (refer to the
paper presented below), shows the effect of bank angle on actual VMCA.
It in fact shows that a higher speed is required to maintain straight flight
with an inoperative engine, for the given size of the vertical tail at
different bank angles.
The figure also shows that if a turn is initiated while
the airspeed is close to the AFM-listed VMCA, as is the case
during takeoff, go-around and initial climb, then the actual VMCA
will increase above the indicated airspeed which leads to the loss of
control; the vertical tail is simply not big enough to prevent the loss of
control. Turning at low speed and high power settings is therefore
extremely dangerous. A catastrophe following the failure of engine and
while the other engine(s) are producing max. thrust can only be prevented by
maintaining straight flight while keeping the bank angle 3 to 5 degrees (as
opted by the manufacturer) away from the inoperative engine, because this is
how the vertical tail was designed and dimensioned and how VMCA
was measured. A lower thrust setting than max., or a forward cg of
course reduces the asymmetrical thrust moment or increases rudder power and
hence decreases the actual VMCA. This is why not all
takeoff or go-around engine
failures end in a disaster.
Deficiencies
in manuals. AvioConsult also concluded that the definition of VMCA, that is used in textbooks,
in Flight, Operating and Training Manuals, was
often copied straight out of Aviation Regulations (FAR/ EASA/CS 23.149 or
25.149 or equivalent).
From the manufacturers point of view, this might look safe and also acceptable to the
authorities that have to approve the manuals. However, FAR and EASA/CS Part
23 and 25 are for the certification of an airplane and definitely not for
operations with the airplanes. The inappropriate copying of the
definition of VMCA out of Aviation Regulations leads to incorrect and incomplete definitions of VMCA in
the manuals used by operational pilots and hence leads to inappropriate crew response to propulsion system malfunction.
In most Flight Manuals of multi-engine airplanes (big and small), there are
at least four (4!) deficiencies and imperfections
in the definition of VMCA and are two important
conditions missing as a consequence of the inappropriate copywork. These deficient and imperfect definitions have led and will lead again to a dangerous misunderstanding of air minimum
control speed VMCA and of takeoff safety speed V2 by most, if not all, multi-engine rated
pilots and will continue to lead to catastrophic accidents in case an engine fails during
takeoff or go-around, or during maneuvering while an engine is inoperative.
Conclusion. During the search
for the real cause of engine failure related accidents, AvioConsult
reviewed many accident
investigation reports, flight manuals and training material and noticed that
VMCA was almost never considered a factor as cause of such an accident because it was unkown to the investigators (and to
the mishap pilots) that the actual VMCA changes considerably with
bank angle and engine thrust. Comments on a limited
number of these accidents are presented on a separate page; please click here.
AvioConsult decided to do something for the prevention of airplane
accidents after engine failure and wrote papers and reports that are presented
and downloadable below.
One of the recommendations is to include a warning with VMCA (and V2) data in each and every Flight and/ or Operating Manual of multi-engine airplanes and in student pilot textbooks. The warning could look like this:
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WARNING The flight manual listed VMCA
and the pre-flight calculated V2 are valid only while maintaining the
same bank angle that was used by the manufacturer to design the
vertical tail and to determine VMCA Keeping the wings level after engine failure increases the actual VMCA by 8 to 35 knots (depending on the airplane type and power setting) above the Flight Manual listed VMCA which might lead to an immediate and unexpected loss of control from which recovery will not be possible if the altitude is low. |
Control
after Engine Failure
This report,
first called 'Prevention of Airplane Accidents after Engine Failure',
presents almost all there is to know about flight with an
inoperative engine and was prepared
using the knowledge that formally trained
experimental test pilots and flight-test engineers have on the subject
and on the proper flight test techniques to determine VMCA
and is
intended for pilots, instructors, teachers, aviation authorities,
accident investigators, etc.
The report (28 pages, 25 figures) includes
all of the following subjects:
airplane control after engine failure;
variable factors that influence minimum control speed VMCA;
flight-testing VMCA and the airplane configuration used;
the apparent safety of takeoff safety speed V2;
imperfections and deficiencies in flight manuals and text books on the subject engine failure and flight with an inoperative engine.
Included in the report are many ready-to-copy recommendations to improve:
engine emergency procedures;
flight manuals;
student pilot textbooks;
engine-out training;
primary flight display.
AvioConsult believes
this report was necessary because too many unnecessary accidents are happening
following the failure of an engine.
This report is available for download:
| Download this report | Return to downloads page |
In case
you are interested, a two to four hour
lecture
on the subject of Airplane Control After Engine Failure is
available, please contact
AvioConsult.
Besides to the EASS of the Flight Safety Foundation,
a lecture was recently presented to
the airplane accident investigators of the Dutch Transportation Safety
Board. An earlier, less extensive version was presented in a meeting
with the Engine and Propeller Directorate
of the FAA and ALPA, to the Flight Safety Committee of the Dutch Airline Pilot
Association, to the Netherlands Association of Aeronautical Engineers, the Air Forces Flight
Safety Committee Europe and to
Air Force and Navy. Thank you for wanting to learn
more about making aviation an even safer means of transportation.
Please
refer to the accidents page
for a number of accident descriptions.
The Effect of Bank Angle and Weight on VMCA
In the report presented above, graphs showing the effect of bank angle and weight on VMCA and on takeoff safety speed V2 are included. These graphs were calculated using a prediction method that is also used by experimental test pilots and flight test engineers before beginning the flight-tests to determine VMCA in order to learn about limitations, etc. that might be encountered during the test. This paper presents the prediction method and includes a few data figures. This method can be used for all multi-engine airplanes, provided the required stability derivative data are available.
This
report
is available for download:
| Download this report | Return to downloads page |
Staying Alive with a Dead Engine
A paper by AvioConsult, presented at the European Aviation Safety Seminar (EASS) of the Flight Safety Foundation (FSF) on 14 March 2006 in Athens, Greece.
Abstract: During flight-testing multi-engine airplanes while an engine is inoperative, the manufacturer may opt to use a small bank angle (max. 5 degrees away from the inoperative engine) to determine the minimum control speed VMCA that is to be listed in flight manuals as an operational limitation and that is used to calculate takeoff safety speed V2.
However, any deviation
from this bank angle increases the actual VMCA.
Keeping wings level increases VMCA
already by some 8 kt; banking into the dead engine increases VMCA
much more. But the airline
pilot does not get to know which bank angle was used and should be
maintained for the listed VMCA
to be valid, because there is no requirement in FAR's 23 and 25 to
present this bank angle in the Airplane Flight Manual. Manuals
currently concentrate on the loss of performance after engine failure,
not on maintaining control.
Many pilots also believe that takeoff safety speed V2 is a
safe speed after engine failure. V2 however, is calculated using VMCA (and VS). If the pilot
does not maintain the bank angle that was used to determine VMCA,
then the actual VMCA
might increase to a value higher than V2: control will be
lost.
Not maintaining the bank angle used to determine VMCA after engine failure during takeoff or go-around
can make the difference between life and death.
This paper is a much abbreviated version of the report presented above.
This paper is available for download:
| Download this paper | Return to downloads page |
Imperfections in FAA and EASA Regulations
A report by AvioConsult that resulted from the research for the reports and papers presented above. It presents and explains many errors found in aviation regulations and includes ready-to-copy suggestions for improvement.
This report is available for download:
| Download this report | Return to downloads page |
In case you are interested in having AvioConsult verify and/ or improve your Flight Manuals, engine emergency procedures, textbooks, accident investigation reports, etc. on the subject, please contact AvioConsult.
Comments and downloads on several accident investigation reports and training material, can be found here. The comments were written using the knowledge from the paper and the report presented above.
Please inform friends and colleagues who need to know about this by forwarding the website address of this site to them by clicking here.
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