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Path: uunet!usna!dfr
From: dfr@usna.NAVY.MIL (Prof. David F. Rogers)
Newsgroups: rec.aviation
Subject: Turnback after engine failure
Keywords: turnback, engine failure
Message-ID: <352@usna.NAVY.MIL>
Date: Tue, 23 Oct 90 12:24:53 PDT
Organization: U.S. Naval Academy, Annapolis, MD
Lines: 230

G'day,
 
In a recent issue of Flying magazine (September 1990, pp. 94--97) Peter
Garrison liberally abstracted from my student Brent Jett's turnback
study. The article rather muddied the waters. In response I sent the
article given below to Flying editor J. Mac McClellan for consideration
on 28 August. To date I have not had even an acknowledgement of receipt.
Oh well! Below I share it with rec.aviation.
 
In addition, the most recent issue of Aviation Safety (October 15)
has an article by Bill Kelly entitled Surviving a Power Loss on Takeoff
in which in my opinion he basically give all the wrong advice including
conducting a number of experiments at the wrong conditions and thus
drawing incorrect conclusions from his results.
 
Again I caution you to go out and practice this stuff AT ALTITUDE
with a qualified and knowledgeable CFI on board before having to
do it in an emergency. Further, don't use the turnback technique
if there is a nice smooth field directly ahead of you.
 
Dave Rogers
Bonanza N2255A
 
_________________________________
 
You Can Go Home Again---Revisited
Copyright 1990 David F. Rogers,	All rights reserved.
 
David F. Rogers, Phd
Professor of Aerospace Engineering
 
 
Peter Garrison's article ``You Can Go Home Again'' in the September 1990
issue was quite interesting. However, some corrections, comments and
additions are in order. First I conceived, designed and then Midshipman 1/c
Brent W. Jett conducted the experimental research project entitled `` The
Feasibility of Turnback from a Low Altitude Engine Failure During the
Takeoff Climb-out Phase'' under my direction. Professor Bernard `Bud'
Carson, who is a long time colleague and friend as well as one of the finest
aerodynamists that I know, contributed the theoretical analysis of the
optimum conditions for execution of the decelerating descending turn during
the course of the research. Consequently he is listed as a co-advisor on the
project.
 
It is important to realize that the research was conducted in a modified
GAT-IVS simulator. This simulator does not have a visual display. The only
visual clues available to the pilot were a horizontal line and the letters
N,S,E, and W for North, South, East, and West painted on the room walls.
Thus, the flights were `essentially' performed on `instruments'. This also
accounts for the somewhat arbitrary, but practical and reasonable, criteria
for a safe landing. These were defined as: maximum decent rate less than
2500 fpm, rate of decent at touchdown less than 500 fpm, wings level
+-5 degrees below 100 feet altitude, turn of at least 175 degrees
completed above 100 feet altitude and maximum bank angle less than
55 degrees. The tests were performed in the no wind condition.
 
Jett's paper recommends that the turn be flown at a 45 degrees bank angle
`just above stall'. Garrison wonders what `just above stall' means.
Theoretical analysis shows that the optimum velocity for the turn is stall
velocity. (Actually it shows that the maximum lift coefficient is optimal.)
Recommending that a pilot fly the aircraft at stall velocity would be
foolish. Consequently, Jett's paper recommended `just above the stall
velocity'. Since most stall warning horns are set between 5 and 10 mph (kts)
above stall, practically this means with the stall warning horn blaring or
the stall warning light bright.  If a horn is fitted, then the aural input
`somewhat' reduces the requirement to scan the air speed indicator and thus
reduces pilot work load.
 
Garrison states that tests were conducted ``.... and with `full rudder'
in recognition of the fact that a desperate pilot might try to hurry his
..... turn with extra rudder.'' Although as pilots we recognize that this
statement is true, it is NOT the fundamental reason the `full rudder' tests
were conducted. Consider that the fundamental objective of the turnback
maneuver is to point the nose of the aircraft at the end of the runway.
Using full rudder induces a high yaw rate about the vertical axis of the
aircraft. Thus the aircraft's nose more quickly `points at the end of the
runway' with potentially less loss of altitude and less distance from the
end of the runway then in a coordinated turn. Unfortunately, as anticipated,
the high yaw rate reduces the effective air speed over the inboard wing, the
inboard wing stalls and a classic stall/spin occurs. In fact, if there were
sufficient time and/or altitude for the spin to develop fully it would be
inverted because of the increased rolling moment caused by the increased
effective speed and hence lift on the outboard wing.
 
Garrison's comment that ``..... the issue here is minimum sink rate, not
L/D ratio, .....'' is incorrect. The problem, as shown by the theoretical
analysis, is not that simple. In fact, as shown by the simulator data the
sink rate is quite high; nearly a 1000 fpm for the successful flights. The
real issue here is energy management.  Briefly, the potential energy stored
in altitude must be optimally traded to supply the kinetic energy required
to overcome drag while executing a 180+ degree descending turn. The
theoretical analysis says that the optimal conditions to do this are a
45 degree banked turn at maximum lift coefficient (stall velocity). The
experimental simulator studies confirm the theoretical calculations.
 
Garrison wonders whether an angle-of-attack indicator would be a better
control device than an air speed indicator. Of course it would. The objective
here is to fly the aircraft as close to maximum lift coefficient (stall
angle-of-attack) as possible. An angle-of-attack indicator is a much better
indicator of stall angle-of-attack than an air speed indicator. However,
consider that a stall warning device is in fact a crude angle-of-attack
indicator and is thus (perhaps) a better control device for the
maneuver under consideration than the air speed indicator. Unfortunately, all
these devices, including an angle-of-attack indicator, are basically
designed to give correct indications ONLY in coordinated flight. None
of them work well in uncoordinated flight. For example, if the stall warning
device is mounted on the left wing, then in an uncoordinated turn to the
right using excessive right rudder the left wing may be operating below the
stall angle-of-attack while the right wing is fully stalled. The stall
warning device does not indicate a stall, the pilot increases
angle-of-attack and a stall/spin results. Conversely, in an uncoordinated
turn to the left with excessive left rudder the stall warning device
indicates the incipient left wing stall, the pilot reduces angle-of-attack
and the stall/spin is avoided.  Practical angle-of-attack indicators have
similar problems. Perhaps two stall warning devices are in order, one
on each wing.
 
Garrison wonders what the effects of wing loading, weight, and drag
(cleanliness) are on the results. These parameters have no effect on the
optimal conditions, i.e., 45 degree bank angle at the maximum lift
coefficient (stall angle-of-attack).  However, as every pilot knows, they
certainly affect the velocity at which maximum lift coefficient occurs in a
45 degree bank. They also affect the altitude loss during that turn.  There
are no surprises here. A heavy aircraft with a high wing loading, a low L/D
ratio (glide ratio), in a dirty configuration requires a higher starting
altitude to successfully complete the maneuver than a light, clean aircraft
with a high L/D ratio (glide ratio). Each different aircraft (and each
pilot) requires a different starting altitude to successfully complete the
maneuver. Incidentally, this turnback maneuver is well known to sailplane
pilots. It is the standard maneuver when the tow rope breaks. For a
sailplane, a typical starting altitude is 200 feet.
 
The results of the simulator study were subsequently verified by flight
tests. These tests were not reported in Jett's paper. The flight tests were
conducted using a Beech Bonanza F33A, a Piper Cherokee 140 and a Cessna 172.
Tests in all three aircraft were conducted at an altitude of approximately
3000 feet.  These flight tests verified the results of the simulator study
for both 30 degree and 45 degree bank angles using a velocity just above
stall for a 180+ degree turn.
 
A limited series of low level flight tests each of which included an actual
take-off were conducted with the Cessna 172. Lift off occurred at
approximately 2000 feet along a 3000 foot runway. Winds were calm. Weight
was approximately 2150 pounds. Engine failure was simulated at 500 feet agl
by reducing power to idle with full carburetor heat applied. An immediate
30 degree banked turn at approximately 10 mph above stall velocity was
initiated. The required heading change to return to the departure end of the
runway was approximately 210 degrees. Upon completion of the turn the
velocity for maximum L/D ratio (best glide ratio) was established.  No flaps
were used.  In each flight test the aircraft would have landed between 200
and 300 feet short of the runway end. At approximately 100 feet agl power
was applied and the aircraft landed.
 
Based on the results of the simulator study, Jett properly concluded that
``Turning with 30 degree bank, coordinated rudder, at an air speed slightly
above stall, will yield the best combination of performance and safety.'' In
light of the limited series of low level flight tests described above, this
conclusion should be modified to more nearly reflect the theoretical optimum
requirements, i.e., 45 degree bank angle at the maximum lift coefficient
(stall angle-of-attack). The fundamental reason is that a 45 degree bank
angle results in a significantly smaller radius of turn which decreases both
the distance from the runway end extended and the distance to the side of
the runway at the end of the turn. The simulator studies indicate that there
is a negligible difference in altitude lost during the turn (2 ft. on
average out of 340 feet) and only a minor difference in rate-of-sink (73 fpm
on average) at completion of the turn between a 30 degree and 45 degree
bank angle. The simulator study also shows that a 30 degree bank
angle places the aircraft an estimated 225 feet further from the runway than
with a 45 degree banked turn. Notice that this is about the distance that
the aircraft was short of the runway end during the low level flight tests.
Also notice that the low level flight tests were conducted in zero wind
conditions as were the simulator studies. This is the worst case. Provided
that the aircraft is turned into any crosswind component, a headwind or
crosswind will help the aircraft reach the end of the runway.
 
What about safety? Garrison correctly stated that the simulator study showed
a 75% success rate for the 45 degree banked and a 95% success rate for
the 30 degree banked turns ON THE FIRST TRY.  What Garrison failed to
mention is that in the simulator study each of the unsuccessful pilots of
the 45 degree banked turn was allowed two additional attempts to
successfully complete the maneuver. Except in two cases, every pilot
successfully completed the maneuver. That's a better than 90% success rate.
Of the two pilots that failed to successfully complete the maneuver, one was
a student pilot and the other a less than 100 hour new private pilot.  The
simulator study indicates that training makes this maneuver reasonably safe.
This should not be surprising.  Sailplane pilots are required to train for
this maneuver and to perform the maneuver to successfully complete the check
ride for their rating. Why not power pilots? Power pilots are required to
train for and to demonstrate level stalls, departure stalls, approach
stalls, forced landings, etc. to qualify for the private pilot rating.
There's a lot of hostile terrain at the end of the runway, e.g., at island
airports, mountain valley airports, city airports and almost any airport at
night.  If the engine quits on climb out, frequently, the best choice of
terrain for a forced landing may be behind you, on the runway. Let's train to
be able to use it.
 
Garrison raises the issue of overshooting the runway when taking off into a
strong headwind. Let's ROUGHLY look at this possibility.  To do so it is
necessary to make some assumptions. Here, consider a typical light single
engine GA aircraft departing from a 3000 foot runway. Lift off occurs at
1200 feet and a climb at an average speed of 75 mph is established with an
average rate-of-climb of 600 fpm. With no head wind component, after 50 sec.
the aircraft will be at 500 feet agl and 5500 feet from the take off point
or 3700 feet from the take-off end of the runway. Now assume that the
turn leaves the aircraft at the same distance from
the end of the runway at the completion of the turn at 200 feet agl with the
air speed at 80 mph (best glide speed) with a sink rate of 500 fpm.  That's a
lot of assumptions but they are reasonable and this IS a rough
calculation. In the 20 seconds it takes to lose the 200 feet of altitude, the
aircraft travels approximately 2800 feet and will land 900 feet short of
the runway. This rough result QUALITATIVELY corresponds to the low
altitude experimental flight test results obtained with the Cessna 172 for
zero wind. Allowing 30 seconds to complete the turn the
entire flight is only 100 seconds long. In 100 seconds a headwind component
of 27 mph is required to carry the aircraft beyond the beginning of the
runway. If this occurs, the pilot can use flaps or slip the aircraft to
increase the rate of decent, which also decreases the flight time and hence
the time for the wind to act on the aircraft, and thus land the aircraft
on the runway. Even if a long fast landing occurs, it is better to
dribble off the end of the runway or to deliberately ground loop the
aircraft at low speed then to land into unknown or possibly hostile terrain.
Notice also that even a modest headwind significantly contributes to the
probability of making the end of the runway.
 
All aspects of flying, especially emergencies, require the pilot to
make a continuing series of choices. The more informed the choices,
the more successful the flight. Making the right choices requires both
thought and training. Let's go out and safely and thoughtfully train for this
emergency situation just like we train for many others.