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Revisiting the HS Trident Deep Stall accident in 1966

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Revisiting the HS Trident Deep Stall accident in 1966

Old 4th Aug 2018, 21:43
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Originally Posted by JammedStab
I have to admit, I was surprised to discover that the 727 does not have a stick pusher to ensure that there is never a deep stall. ATR has one though.
JammedStab the ATR is most likely fitted with a stick pusher in order to prevent spin entry rather than for deep stalls.
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Old 5th Aug 2018, 01:18
  #22 (permalink)  
 
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dixi, don't know about later, but initial introduction saw the DAN AIR 727 fitted with stick pusher, stick nudger and stick shaker to offer full stall protection.

ATR 42 Model: 400/500 manual
A stick pusher and a stick shaker are provided, preventing the aircraft from reaching a critical angle of attack. When the detected incidence becomes too high, the MFC sends a signal to an electric actuator which shakes the control column at stall alert thresholds.If angle of attack keeps increasing, a further threshold is reached and the MFC activates the stick pusher ; the complete pitch control linkage assembly is pushed forward. Note : There are two stick shakers, one for each control column but only one stick pusher actuator located on the captain pitch channel. In case of pitch uncoupling when the pusher triggering angle of attack is reached, only the captain control column is pushed forward.
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Old 5th Aug 2018, 03:46
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Typically, there is only one reason any aircraft is fitted with a stick pusher - the type cannot be shown to comply with the stall handling characteristics requirements of the applicable certification standard without it.
The stall handling deficiencies may included, deep stalling, unacceptable longitudinal handling or stability on the stall approach, unacceptable levels of "roll off" at the stall.
While, often, unacceptable characteristics may be corrected with aerodynamic fixes, that may be the more expensive option. It may be cheaper just to fit a known acceptable stick pusher system than to embark on a stall development programme with an uncertain outcome. Also, the aerodynamic fix may result in an unacceptable increase in stall speeds from a performance viewpoint.
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Old 6th Aug 2018, 11:11
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Originally Posted by G0ULI
There is only one possible method of recovery from a deep stall, which involves banking the aircraft 90 and pointing the nose down. Given enough altitude, sufficient airflow will develop over the elevators to enable the aircraft to be recovered. Potential pitfalls are overspeed, entering a flat spin, disruption of airflow into the jet engine intakes and entering another high speed stall while attempting a recovery pullout. This really is a last throw of the dice!
for an rapid airspeed loss and a pitch up the procedure is to turn near knife edge and allow the nose to drop then recover from the subsequent spiral dive...Nothing as said earlier will work you out of a deepstall
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Old 6th Aug 2018, 11:18
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Originally Posted by Pugilistic Animus
for an rapid airspeed loss and a pitch up the procedure is to turn near knife edge and allow the nose to drop then recover from the subsequent spiral dive...Nothing as said earlier will work you out of a deepstall
The ATR can be recovered from a deep tail stall by lowering flaps (to 15) should you manage to get into one.
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Old 6th Aug 2018, 11:54
  #26 (permalink)  

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Originally Posted by dixi188
IIRC the first 2 B727s on the UK register (Dan Air) had stick pushers fitted as a CAA requirement. Later aircraft did not have them fitted.
They had little square holes in the centre panel which, I recall, was for the accumulator warning lights for the pushers. The CAA boss at the time (Davies - he of the famous book) had a thing about pushers. As soon as he went, they were ripped out.

The JATO aircraft was easily identified by the circuit-breaker panel - the JATO pair of cbs was still placarded. More obvious was the huge strengthening panel on top of the fwd fuselage - the JATO had set fire to the aircraft and it was extensively re-built.
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Old 9th Aug 2018, 16:06
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Originally Posted by safetypee
Ken, the situation may not be quite so absolute as you suggest. A stalled wing does not mean no lift, just insufficient lift for the conditions; see Cl vs AoA charts.
A deep stall requires a high AOA. So high that the tail is blanked by the turbulent wing flow. At that high of an AOA the wing is fully stalled and there is effectively no roll control. It is why during flight testing of deep stall conditions that the test aircraft is fitted with a drogue parachute to break the stall condition. You can't fly out of it. The roll and pitch maneuver is only effective to PREVENT a normal stall from progressing to a deep stall condition. If you've stalled, then do not quickly apply the correct procedures to prevent it from progressing to a deep stall, and find yourself in a deep stall, kiss your butt good bye.

During stall testing of the C-17 it was equipped with a recovery parachute and an emergency escape trunk. Even the recovery parachute might fail to break the stall in which case the pilots would use the emergency escape trunk to exit the aircraft and parachute to safety.

Last edited by KenV; 9th Aug 2018 at 16:17.
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Old 9th Aug 2018, 21:04
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KenV, I maintain my alternative view, #14. I would accept that the exact nature of the stall will, as ever, depend on the aircraft type, wing characteristics, control system, trim and the range of cg; and an ‘otherwise healthy aircraft’ (MFS #15).

The residual lift at ‘high’ AoA will be proportional to the wing characteristics, depending on how fast Cl falls away from Clmax with increasing AoA. The lack of either roll or pitch control generally discounts any residual ability of the control surface to ‘affect the airflow’, even although it is disturbed airflow, there still is airflow.

Other discounted factors might include any tendency for natural roll-off during the stall entry, span-wise airflow or disturbance which might affect roll, e.g. engine pod or pylon vortex shedding vs AoA or small values of sideslip.
Differing views might originate from specific type experience, theory or practice. Although not familiar with the C17, I suspect that my civil type experience is remarkably similar.

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Old 10th Aug 2018, 04:41
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Deep stall doesn't require extreme angles of incidence. It just requires sufficient incidence that the nose down pitching moments available to the pilot are insufficient for recovery.
I would be surprised if AF447 didn't achieve incidence angles far beyond those at which some T tailed types would be considered deep stalled, but it demonstrated roll oscillations showing that there were cyclic variations in lift across the wing.
During the early days of deep stall research it was found, for at least one transport aircraft type, that apparently benign sink in a level attitude could result in deep stall alpha with the pilot unaware of what was occurring.
Of course, many cases of deep stall do require extreme alpha to "lock in".
With regard to aerodynamic controls on a stalled wing, they are often shown to be effective, albiet with, probably, hugely reduced response.
I have no idea how "deeply" stalled the Hunter wing was during spinning, but on a daily basis demonstrations were made of the power of the ailerons to either recover from a spin, despite full pro-spin controls being held with the rudder and elevators, or prevent spin recovery despite full anti spin controls being held with the rudder and elevator.
This was a demonstration of the rolling moments the ailerons could produce which cross coupled to produce significant pro or anti spin yawing moments.

Last edited by zzuf; 11th Aug 2018 at 02:57.
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Old 10th Aug 2018, 09:22
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Originally Posted by safetypee
dixi, AFAIR the Trident accident was during a pre-delivery production test flight, thus additional experimental test-flight devices were not fitted.

Nowadays where pre-delivery testing requires a stall check either with or without stick push (stall ident) enabled, an additional glare-shield stall panel is fitted showing AoA calibrated for stick shake / push. Preflight the AoA vanes and indicator are crosschecked, so that if when inflight the shake / push do not occur at the expected AOA then the test is stopped.
The BAe 146 used such a panel, which may also have been approved for training stalls, providing the shake / push was enabled and checked / calibrated pre-flight.

Mmmm... Twice, in another life I have terminated stall tests during a certification validation when the stall ident/stick pusher didn't operate at the scheduled speed. As a flight test team, our flight test engineer concentrated almost exclusively on the ASI during the deceleration. We allowed 1kt below the scheduled speed - if it were possible to read the ASI to that accuracy, before terminating the flight. Very stressful knowing that the pusher is required because of deep stall concerns, and no stall test instrumentation fitted.
These tests can be very demanding to fly. For the stall devices to operate on schedule, the correct approach must be flown. Any errors in approach rate or small G errors will negate the approach.
The TP often has little familiarity with the type and the manufacturer takes a dim view of the termination of flights - no pressure.

Last edited by zzuf; 10th Aug 2018 at 10:14.
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