Understanding the Stall-recovery Procedure for Turboprop Airplanes

36 Pages·2005·1.77 MB·English

by  Flight Safety Foundation

Flight Safety
D I G E S T
APRIL 2005
Understanding the
Stall-recovery Procedure
For Turboprop Airplanes
In Icing Conditions
Flight Safety Digest
Flight Safety Foundation
Vol. 24 No. 4 April 2005
For Everyone Concerned With the Safety of Flight
www.fl ightsafety.org
OFFICERS AND STAFF In This Issue
Chairman, Board of Governors Amb. Edward W. Stimpson
President and CEO Stuart Matthews
Executive Vice President Robert H. Vandel
General Counsel and Secretary Kenneth P. Quinn, Esq.
Understanding the Stall-recovery
Treasurer David J. Barger
Procedure for Turboprop Airplanes in
ADMINISTRATIVE Icing Conditions
Manager, Support Services Linda Crowley Horger
Current pilot training typically emphasizes powering through a
FINANCIAL stall recovery with no loss of altitude. Nevertheless, when fl ying
a turboprop airplane that has accumulated ice, lowering the
Director of Finance nose to reduce angle-of-attack is imperative. Here’s why.
and Administration Juan G. Gonzalez
1
Accountant Millicent Wheeler
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Production Coordinator Karen K. Ehrlich LI Models of human error derived from engineering and
Production Designer Ann L. Mullikin experimental psychology are inadequate to understand
Production Specialist Susan D. Reed 22 the complex interaction among technology, operators and
organizational systems, the author says.
Librarian, Jerry Lederer
Aviation Safety Library Patricia Setze
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Q-Star Program Administrator Robert Feeler
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Manager of Aviation
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Cover photo: © Copyright 2005 Getty images Inc.
Supercooled large
droplets froze instantly
on contact with a side
window in the airplane
cockpit during an
icing test flight.
(Source: John P. Dow Sr.)
Understanding the Stall-recovery
Procedure for Turboprop
Airplanes in Icing Conditions
Current pilot training typically emphasizes powering through a stall recovery with no loss
of altitude. Nevertheless, when flying a turboprop airplane that has accumulated ice,
lowering the nose to reduce angle-of-attack is imperative. Here’s why.
— JOHN P. DOW SR.
M
ost encounters with icing condi- required for stall recovery will better prepare
tions in turbopropeller-driven a turboprop airplane pilot to respond to one
(turboprop) airplanes are rela- of these infrequent but very dangerous icing
tively benign and demand little encounters.
more than promptly activating the airplane’s ice-
protection systems and finding an ice-free alti- An airplane upset is defined by the Airplane
tude. Nevertheless, there have been encounters Upset Recovery Training Aid as including the fol-
with icing conditions that have caused rapid and lowing unintentional conditions: “Pitch attitude
adverse airplane responses, including stalls that greater than 25 degrees nose-up; pitch attitude
have led to airplane upsets and loss of control. greater than 10 degrees nose-down; bank angle
A broader understanding of what might be greater than 45 degrees [or] within the above
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parameters but flying at airspeeds The researchers said that a primary factor in suc-
inappropriate for the conditions.”1 cessful recovery was nose-down elevator input.
The training aid says that specific Their report said:
values may vary among airplane
models. The pilots appeared to respond in accordance
with their training for excessive bank and stall
Loss of control is defined by the recovery, but they did not implement corrective
European Joint Aviation Authorities actions uniquely required for icing-induced roll
(JAA) Safety Strategy Initiative as “a and uncommanded control movement. These
situation in which the crew fail[s] two types of recoveries require different respons-
to maintain/regain control of an es: Normal stall-recovery training (which trains
aircraft. This can result from external pilots in recovering from the approach to stall)
factors, such as icing or mechanical emphasizes applying maximum power and
failures.”2 minimizing loss of altitude. In contrast, recov-
ery from icing-induced rolls and more complete
The U.S. Commercial Aviation stalls requires trading altitude for airspeed.
Safety Team has a similar, but
broader, defi nition: “Loss of con-
Fly Like You Train (Usually)
trol refers to accidents resulting
from situations in which the pilot
should have maintained or regained aircraft Most ice-related stalls and upsets occur in in-
control but did not.”3 strument meteorological conditions (IMC).
Instrument interpretation (e.g., when the airplane
Examination of digital flight data recorder is in an unusual attitude) requires skills that pilots
(DFDR) data from turboprop airplanes involved typically have not developed in training or from
in ice-related loss-of-control accidents has shown experience. Moreover, because an ice-related stall
some common characteristics. For example, in typically occurs at a lower-than-normal AOA
three fatal accidents that resulted in 134 total (and higher-than-normal airspeed), the stall can
fatalities, the pilots initially did not reduce wing surprise the pilot.
angle-of-attack (AOA) by moving the control
column to the nose-down position early in the Pilots of turbine airplanes typically are trained in
upset sequence. The accidents involved an Avions fl ight simulators to respond to the fi rst indication
de Transport Regional ATR 42 in Crezzo, Italy, of a stall by applying power and maintaining pitch
in 1987;4 an ATR 72 in Roselawn, Indiana, U.S. attitude, with the objective of losing no altitude
in 1994;5 and an Embraer Brasilia in Monroe, during recovery. Thus, in theory, the airplane
Michigan, U.S., in 1997.6 will accelerate to an increased airspeed and a
reduced AOA. The procedure results in recovery
The DFDR data from the three accident airplanes in the simulator; nevertheless, the procedure will
show that AOA either remained close to the angle not always result in recovery in an airplane with
at which airfl ow separation occurred or that AOA fl ight characteristics degraded by ice.
increased, compounding the severity of the upset
and making recovery more diffi cult and unlikely. The training follows U.S. Federal Aviation
Other ice-related incidents from which fl ight data Administration (FAA) practical test standards
were available also involved AOAs that were main- (PTS). The PTS for the private pilot certifi cate
tained or increased. and the PTS for the commercial pilot certifi cate,
for example, specify a “minimum loss of altitude”
The scenario that was involved in the Roselawn during stall recovery.
accident was duplicated in a flight simula-
tor during a study conducted for the U.S. The PTS for the airline transport pilot certifi cate
National Aeronautics and Space Administration and for aircraft type ratings require recovery to
(NASA).7 The subjects for the study were 40 be initiated at “the fi rst indication of an impend-
newly hired airline pilots; fewer than half were ing stall” and to be completed with “acceptable”
able to recover. altitude loss. The JAA standards are similar.
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AFM Procedures Vary Nevertheless, the PTS standards — primarily those
for the ATP certifi cate and airplane type ratings
Some airplane fl ight manuals (AFMs) and fl ight — typically are adhered to during training.
crew operating manuals (FCOMs) include rec-
ommended stall-recovery procedures, although “It doesn’t make much difference what the
the information is not required. The FCOMs for manufacturers recommend, the pilots that fl y
the ATR 42 and the ATR 72, for example, recom- the airplanes must get a check ride from the FAA
mend the following procedure for a “stall without and perform the required procedures,” said Jon
ice accretion”:8 Hannan, former fl ight test pilot for the FAA Small
Airplane Directorate.12
Recovery of stall approaches should normally
be started as soon as a stall alert is perceived: The PTS “acceptable-altitude-loss” standard,
a gentle pilot push [on the control column] which typically is interpreted as zero altitude loss
(together with power increase if applicable) during training, is designed to avoid terrain con-
will then allow instant recovery. tact during stall recoveries at low altitude.
The FCOMs for the ATR 42 and the ATR 72 rec- “Most stalls occur on approach or on takeoff, when
ommend the following procedure for a “stall with you don’t have a lot of altitude to spare; the idea is
ice accretion”: to conserve altitude during recovery,” said Daniel
Meier Jr., aviation safety inspector, fl ight opera-
Recovery of stall in such conditions must be tions, at FAA headquarters.13 “A stall caused by
started as soon as stall warning is activat- icing is extremely hazardous because you cannot
ing or buffeting and/or beginning of lateral conserve altitude by maintaining attitude.”
instability and/or sudden roll-off is perceived.
Recovery will be best accomplished by: a pi-
Trade Altitude for Airspeed
lot push on the wheel as necessary to regain
control; selection of fl aps 15; [and] increase
Adhering to the standard of minimum altitude
in power, up to MCT [maximum continuous
loss ingrained in training has resulted in pi-
thrust] if needed.
lots failing to recover from ice-related stalls and
upsets that have resulted in altitude losses in excess
“To my knowledge, it has always been ATR standard
of 9,000 feet in turboprop airplanes.
policy to recommend a pitch-down command for
stall recovery, as well as applying power,” said Gilbert
Defer, former vice president of fl ight test for ATR.9 Pilots of turboprop airplanes should be taught
“Furthermore, for stall in icing conditions, ATR has that they might need to trade some altitude for
always recommended extension of the fl aps to the airspeed if the airplane stalls during fl ight in icing
fi rst notch as the most rapid and effi cient manner conditions. Research has shown
to reduce AOA dramatically.” that an immediate and complete
recovery from an ice-related stall
Most AFMs and FCOMs, however, do not recom- likely will be accomplished by us-
mend a recovery procedure for an ice-related stall. ing the following:
The manuals for the Raytheon Beechcraft King Air
models, for example, include only a recommended (cid:127) At the first sign of a stall
procedure for recovery from a non-ice-related, — whether activation of the
single-engine stall.10 stick shaker, uncommanded
roll, buffet or other aerody-
Dave Fisher, senior air safety investigator for namic cues — apply nose-
Bombardier Aerospace, said, “There is nothing in down pitch control and level
our manuals [for the Dash 8] about stall recovery the wings while increasing
because there’s no requirement for it to be in there. propeller speed and torque
However, stall-recovery technique is covered dur- until a sufficient increase
ing initial and recurrent pilot training and prac- in airspeed (decrease in
ticed in the Dash 8 fl ight simulator.”11 AOA) is attained. In most
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events, the nose will drop as a consequence you must increase your airspeed, and the only way
of the stall, but it will result in an insufficient you’re going to do that is to drop the nose.”16
decrease in AOA, requiring further nose-down
pitch change. (The unanticipated sensations
Autopilot Masks Cues
accompanying this pitch change might be
uncomfortable for the pilot);
In training, the visual cues or tactile cues of
impending stall that are presented to the pilot
(cid:127) If the nose cannot be lowered, extend the flaps
typically consist of stick-shaker (stall-warning)
from the cruise configuration to the first set-
system activation. Nevertheless, there have been
ting and then lower the nose to increase speed
some events in which the stall and upset oc-
as appropriate for the airplane type and con-
curred at stick-shaker activation or prior to stick-
figuration; and,
shaker activation in ice-contaminated airplanes.
Additionally, other valuable cues such as aerody-
(cid:127) Recover. Retract the flaps, as appropriate.
namic buffet were ignored or misinterpreted as
propeller vibration.
“This is absolutely correct,” Meier
said. “If you are in icing conditions
With the autopilot engaged, the pilot is “out of
and experience a loss of airspeed, a
the loop” in “feeling” the responsiveness of the
need for more power, diminished
fl ight controls. Moreover, as ice accumulates, the
controllability and/or diminished
autopilot might be required to trim the controls
performance, it’s a pretty good in-
against the adverse aerodynamic effects of the ice.
dication that you are picking up ice
An instantaneous and substantial control input
and not shedding it. If, all of a sud-
might surprise the pilot if the autopilot reaches
den, the airplane falls out of the sky,
its trim-force limits and disengages unexpect-
you’ve stalled because of ice, and
edly. The pilot might be similarly surprised by
the recovery should be nose-down,
unexpected control inputs that might be masked
wings level and full power.”
by the autopilot until the autopilot intentionally
is disengaged.
Training specialists at FlightSafety
International, CAE SimuFlite and
SimCom agree. Airplane response and kinesthetic cues to an ice-
related stall can be substantially different from
“[This] is 100 percent correct,” said the simulator-training scenario. An ice-related
Dan Orlando, director of training stall produces less buffet in some airplanes in
at the FlightSafety International some confi gurations than a non-ice-related stall;
Raytheon Training Center.14 “Our in other airplanes, greater buffet occurs or the buf-
recommended procedure for stall in fet begins earlier in an ice-related stall. The cues
the King Air is to lower the nose and add power also can be inconsistent in the same airplane with
simultaneously.” different fl ap settings.
Chris Litherland, manager of CAE SimuFlite’s Hannan said, “The pilot should hand fl y the air-
King Air program and Beechjet program, said, plane in icing conditions that are severe enough
“Although applying maximum power and relaxing to effect a slowdown because the fi rst indications
back pressure may be suffi cient for a normal stall of stall — mushy control feel and/or small oscilla-
recovery, it is logical that in icing conditions, you tions — usually can be felt in time to recover prior
also may need to lower the nose to compensate for to a stall. If a stall occurs, it is vitally important to
the aerodynamic changes the ice has caused.”15 decrease the AOA quickly, push the nose down
or lower fl aps, and apply power to accelerate to a
Charles Parker, coordinator of SimCom’s King Air higher airspeed.”
90–series program, said, “The normal stall recovery
— that is, when you know you’re not in icing con- Parker said that when an ice-related stall begins
ditions — is to try to maintain your altitude and to occur, “you feel the airplane rumble, and you’re
power out of it. But, if you’re in icing conditions, losing altitude. You have to push the nose over
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when you feel that and add maximum available thrust versus torque shows that propeller blades
power. When you do, the airplane will start fl ying begin to stall at the large blade-pitch angles as-
again.” sociated with high torque values, thus producing
substantially less than 100 percent thrust — in
Some turboprop airplanes are equipped with some events, as low as 85 percent thrust.
stick-pusher systems that lower the nose when
AOA reaches a critical (pre-stall) value. Some Moreover, unclearable contamination (i.e., ice
airplanes are equipped with stall-protection on unprotected surfaces) of the propeller blades
systems that reduce the threshold AOA for stick- also might reduce thrust; tests have shown that
shaker activation and stick-pusher activation when unclearable contamination can reduce thrust by
ice-protection systems are activated. The Dash 8 20 percent.
300 and Dash 8 Q400, for example, have modifi -
cations that enable the pilot to select an “increase
Ice Reduces Stall AOA
Ref speed switch,” which will cause the stick-shaker
to activate at a higher speed during fl ight in icing
conditions.17 Icing redesigns the airplane (see photo below).
A review of what can occur to airplane aero-
Ridges of ice formed
dynamics in severe icing encounters improves
on the leading edge
Insuffi cient Excess Thrust understanding of what is required for stall recov-
of the horizontal
ery. The fi rst effect of ice accumulation usually is
Turboprop airplanes affected by ice-induced a reduction of the stall AOA (and an increase in stabilizer, and
drag typically do not have the substantial ex- the stall speed). There is no way for the pilot to mushroom-shaped
cess thrust of large jet transports that is implicit in know what the resulting stall AOA is at any given ice caps formed on
the power-up and maintain-pitch procedure. moment. the vortex generators
below the stabilizer
In addition to increasing drag and causing air- Figure 1 (page 6) shows how lift can vary with
during a research flight
fl ow separation, ice has an adverse effect on the airspeed in unaccelerated flight. The data on
in a de Havilland Twin
propellers. lift coeffi cients were derived from DFDR data
Otter. The ice was
from a British Aerospace ATP that was involved
not a hazard during
During the upset that led to the loss-of-control ac- in an ice-related upset in August 1991. In cruise
cident at Monroe, the crew at one point increased configuration and uncontaminated, the air- the flight. (Photo: U.S.
engine torque 150 percent. Although increasing plane has a maximum coeffi cient of lift (C ) National Aeronautics and
Lmax
torque would seem benefi cial, analysis of propeller — which corresponds to the stall AOA and, in this Space Administration)
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Figure 1
Effect of Ice Contamination on Lift and Drag
1.6
MaximumC (C )clean
L Lmax
1.4
Coefficientoflift(C )
L
1.2
1.0
CD CLmaxwithicecontamination
C/L 0.8
0.6
0.4
Coefficientofdrag(C )
D
0.2
0.0
100 150 200 250 300 350 400
Airspeed(knots)
Source: John P. Dow Sr.
illustration, to the stall airspeed — of nearly 1.6. Ice-related upsets have occurred at the top of
With the ice contamination involved in the inci- descent, most likely when power was reduced
dent, C was 0.9. Thus, the airplane’s normal before the descent was begun. Accompanying
Lmax
stall speed was about 110 knots; the ice-induced power reduction is a reduction of the velocity of
stall occurred when airspeed was reduced to about airfl ow over the wing; the benefi cial effects of prop
140 knots. wash are reduced, and fl ow separation occurs at
a much lower AOA. The preventive measure for
As ice accumulates, drag increases. The increased this situation is to lower the nose, allow airspeed
drag might be evident early in the icing encoun- to increase, then adjust engine power and pitch
ter; however, in some events, including the acci- trim for the descent.
dent at Roselawn, the drag increase was calculated
to be only 5 percent to 10 percent. In cruise fl ight, Even without partial propeller-blade stall, reduced
with the autopilot engaged in altitude-hold mode thrust caused by uncleared ice on the propeller
and with a constant power setting, airspeed will blades combined with increased ice-related drag
decrease and the autopilot will trim the airplane overall is a double penalty for a turboprop air-
nose-up to maintain the selected altitude. If this plane, compared to an uncontaminated airplane
is allowed to continue until AOA reaches the new that has substantial excess thrust. With partial
(lower) stall AOA, fl ow separation will occur. The propeller-blade stall involved, the disturbed fl ow
result can be an upset. fi eld aft of a stalled blade section further degrades
the aerodynamics of the wing.
During climb, increased drag might be evident
by an unexplained decrease in indicated airspeed
Extending Flaps Can Help
and/or rate of climb (vertical speed). These indi-
cations demand immediate action by the pilot; if
increased propeller speed and increased power do An incident involving an upset in a Brasilia on
not return the airplane to safe speeds, an immedi- March 5, 1998, illustrates the effectiveness
ate descent is required. This is an emergency. of lowering the nose to reduce AOA. DFDR data
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recovered by the U.S. National Transportation as the fl ight deck. The following comments by a
Safety Board (NTSB) show that the upset occurred Convair 340 pilot appeared in a 1964 issue of Air
as airspeed decreased in a turn at 10,000 feet. The Line Pilot magazine:
pilot initially was unable to recover by increasing
power (e.g., increasing torque to 100 percent) and We encountered moderate icing climbing up
maintaining pitch attitude. He was able to recover through to on top. … There was a consider-
after extending the fl aps to the approach setting, able amount of runback [ice]. … This really
which increased the lift coeffi cient, and lowering wasn’t any problem. Hasn’t every pilot landed
the nose, which reduced AOA. an airliner with considerable ice or runback
[ice] on it?
The incident illustrates several important points:
This report, by a Saab 340 pilot, appeared in a 1993
(cid:127) The initial power increase to 100 percent issue of the magazine:
Investigators used a
torque and a later, momentary, power increase
broom handle to crack
to nearly 150 percent torque did not increase [The airplane had] a layer of light rime with
the ice on a Saab
airspeed sufficiently to enable recovery; a layer of clear on top. The FO [fi rst offi cer]
queried the captain if blowing the [deicing] 340A’s wing so that
(cid:127) Nose-up pitch trim was used. The control boots was warranted, … which the captain they could gauge the
column was not moved to the nose-down declined. thickness of the ice.
position until the flaps were extended; the The airplane had been
pitch angle did not change substantially while In 1993, a Saab 340A accumulated ice during an substantially damaged
airspeed was low; approach to Hibbing, Minnesota, U.S. The NTSB
during a hard landing
report said that the fi rst offi cer (the pilot fl ying)
at Hibbing, Minnesota,
(cid:127) Before the upset occurred, the airplane was asked the captain if he wanted to “pop the boots”
U.S., in 1993; none
banked about 25 degrees left. It then rolled to remove ice from the wings. The captain said,
of the 31 occupants
about 65 degrees right and about 45 degrees “It’s going to the hangar. I’ll run them on the
left. When the flaps were extended and pro- ground.” A high sink rate developed, and the air- was injured. (Photo: U.S.
peller speed was increased at approximately plane was substantially damaged in the subsequent National Transportation
100 percent torque, the roll oscillation was hard landing (see photo below).18 Safety Board)
reduced substantially even though pitch
attitude was held relatively constant to the
pre-upset value and airspeed had decreased
to approximately 125 knots; and,
(cid:127) In the recovery, the airplane climbed ap-
proximately 700 feet above the altitude at
the beginning of the upset.
Myth of ‘Safe Ice’ Persists
The accident/incident record shows that pilots
sometimes make incorrect or inappropriate
decisions, which might be based on a lack of ac-
curate and thorough knowledge about fl ight in
icing conditions. The assumption is that pilots
are capable of accurately discerning which ice is
likely to be lethal and which ice is not, and how
their airplane will perform in icing.
A persistent myth is that pilots can discriminate
between “safe ice” and “unsafe ice” from visual
inspection, even from a remote vantage point such
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During an icing
test flight, some
ice remained after
activation of the deice
boots of a Mitsubishi
MU-2 in addition to
the uncleared ice on
unprotected areas of
the MU-2’s wing-tip
fuel tank and fairing.
(Photo: U.S. Federal Aviation
Administration)
More recent events show that similar attitudes are Adequate care of deicing boots and the associ-
held by some pilots and suggest that additional ated pneumatic system is important. Damage (e.g.,
education and training are required to correct cuts, tears) and age-related deterioration can re-
Holes from age-related
a misperception that apparently is a product of duce substantially the effectiveness of boots (see
deterioration of deicing
inconsequential experience with ice. photo bottom left).
boots can allow water
to be ingested by the
Modern deicing boots operate at pressures near
Use the Boots
vacuum pressure that 20 pounds per square inch (one kilogram per
holds the boots against square centimeter) and have infl ation tubes ap-
On turboprop airplanes, ice typically is re-
the airfoil. Trapped proximately 1.0 inch (2.5 centimeters) in diameter.
moved from the leading edges of the wings
water can freeze in the Older tubes are nearly twice that size, have four
and the tail by deicing boots that crack it with
times the volume and operate at lower pressures.
pneumatic plumbing
mechanical force; the particles of ice then are car-
and prevent inflation ried away by the airfl ow (see photo upper right).
The current recommended practice of allowing
of the boots. Part of this process occurs during infl ation, and
ice to build to a thickness of 1/4 inch to 1/2 inch
(Photo: John P. Dow Sr.) part occurs during defl ation.
(approximately 2/3 centimeter to one centime-
ter) before activating the deicing boots results in
a higher percentage of ice being removed on the
fi rst cycle of the boots. The 1/4-inch thickness
can be used as a guideline at temperatures close
to freezing — that is, from approximately minus
5 degrees Celsius (C; 23 degrees Fahrenheit [F])
— in which water droplets freeze relatively slowly
on contact with the airplane and therefore contain
relatively little air; thus, the ice is denser. Because
the water droplets freeze relatively quickly at lower
temperatures — below approximately minus 10
degrees C (14 degrees F) — the ice contains more
air and, thus, is more brittle. In addition, at colder
temperatures, the ice adheres more tenaciously
to the boot; the adhesion is less at warmer tem-
peratures. Thus, allowing colder ice to accrete to
a greater thickness (1/2 inch) before operating the
8 FLIGHT SAFETY FOUNDATION (cid:127) FLIGHT SAFETY DIGEST (cid:127) APRIL 2005