Yet MORE on 727 deep stall, plus references to iced pitot incident

From:         rdd@cactus.org (Robert Dorsett)
Date:         01 Apr 93 23:08:58 PST
References:   1 2 3 4
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Some time back, I wrote:

>>All jet engines have a spool-up time from idle, usually under 8 seconds.

It's actually 5 seconds, courtesy FAR 33.73(b).  This has been effective since
at least 1974; dunno about earlier engines.

"From the fixed minimum flight idle power lever position when provided,
or if not provided, from not more than 15 percent of the rated takeoff power
or thrust available to 95 percent rated takeoff power or thrust in not over
5 seconds.  The 5-second power or thrust response must occur from a stabil-
ized static condition using only the bleed air and accessories loads necessary
to run the engine.  This takeoff rating is specified by the applicant and 
need not include thrust augmentation."


Later, Bruce Watson wrote:

>   On a couple of the turbine aircraft I've flow, there was a minimum
>spinup (aka spool-up) value on final. This was usually measured as one of
>the turbine parameters (on some turbines it was 55% on N1). The idea was
>both to be able to get quicker time to full (in the event of a go-around,
>or to cope with some sort of wind shear situation), but also to get rid
>of some of the throttle lag on the approach. This last use is particularly
>important to avoid making large throttle movements, chasing the glide slope
>all over the place.

More neurons jogged loose: this is probably to keep the surge bleed 
valves closed.  At low engine speeds, a surge valve opens, which is used
to control engine loads during startup.  Spool-up time from flight idle
may have to include the time to clean up.  On the 727, this should be 
around 40% N1, or ~65% N2.  Okay, I stand corrected again! :-)


>I didn't catch the original part of this 727 deep stall thread, but I _think_
>I remember a 727 deep stall accident (the following is entirely from memory,
>but I read about it some years ago in my Canadian aviation safety bulletin):
>   One some cargo flight, the Captain and FO decided to allow the (type-rated
>but rusty) FE flight the departure. As it turned out, they somehow forgot the
>pitot heat. During the climbout, the pitot iced up, and with the static source

This sounds like:

Crash date: December 1, 1974
Type: Boeing 727-251
Operator: Northwest Airlines
Where: Theills, New York
Ident: N274US
Report No: NTSB-AAR-75-13
Pages: 31
Date: August 13, 1975

About 1926 EST on December 1, 1974, Northwest Airlines Flight 6231, a 
Boeing 727-251, crashed about 3.2 nmi west of Thiells, New York.  The 
accident occurred about 12 minutes after the flight had departed John F. 
Kennedy International Airport, Jamaica, New York, and while on a ferry 
flight to Buffalo, New York.  Three crewmembers, the only persons aboard 
the aircraft, died in the crash.  The aircraft was destroyed.

The aircraft stalled at 24,800 feet MSL and entered an uncontrolled 
spiralling descent into the ground.  Throughout the stall and descent, 
the flightcrew did not recognize the actual condition of the aircraft and 
did not take the correct measures necessary to return the aircraft to level 
flight.  Near 3500 feet MSL a large portion of the left horizontal stabilizer 
separated from the aircraft, which made control of the aircraft impossible.

The National Transportation Safety Board determines that the probable cause 
of this accident was the loss of control of the aircraft because the 
flightcrew failed to recognize and correct the aircraftUs high-angle-of-
attack, low-speed stall and its descending spiral.  The stall was 
precipitated by the flightcrewUs improper reaction to erroneous airspeed 
and Mach indications which had resulted from a blockacge of the pitot heads 
by atmospheric icing.  Contrary to standard operational procedures, the 
flightcrew had not activated the pitot head heaters.


------------------------

No reference is made to the F/E in the F/O position, and the F/E didn't
have a type rating.

On page 14 of the report:

"The B-727 longitudinal controls ystem is capable of developing the
noseup pitching moments needed to obtain angles of attack much higher than 
those associated with stall.  For an aircraft having the same weight, 
CG location, and stabilizer trim setting as N274US, the manufacturer's
analysis showed that an angle of attack of approximately 37 degrees could
be attained if a continuous pull force was exerted to hold the control
column aft.

"Like other aircraft which have horizontal stabilizers located near or on 
top of their vertical stabilizers, the B-727 does pass through a range of high
angles of attack where longitudinal instability occurs.  This instability
causes the aircraft, when no control force is applied, to pitch to even higher
angles of attack.  Longitudinal instability is caused by downgraded horizontal
stabilizer effectiveness when the aircraft's atttiude is such that the horizon-
tal stabilizer is enveloped by the low-energy turbulent air in the wake from 
the wings.  When these high angles of attack are reached, a push force on the
control column is required to reduce the angle of attack.  For a B-727 with an
aft CG location and stabilizer trim in the cruise range, wind tunnel data
show that a nosedown pitching moment will decrease the angle of attack and
stall recovery can be attained by applying push forces to the control column."


It goes on to note that the crew apparently thought they were in a stall
all the way down, due to the erroneous airspeed indications, but DFDR data 
showed that they had actually recovered from the stall and were in an 
uncontrolled spiral.




---
Robert Dorsett
rdd@cactus.org
...cs.utexas.edu!cactus.org!rdd