When after lift-off should the red stick shaker tape appear?

[Hardy Heinlin]

Good evening,

on the real 744 the red stick shaker speed on the PFD moves off-screen after touchdown. However, vice versa after lift-off it doesn't reappear immediately (source: BA sim). It's delayed by a certain condition. Does anybody know what the condition is? It's not the gear lever signal.


Regards,

|-|ardy

[emerydc8]

During takeoff, there is no amber band displayed until flap retraction has been initiated. Maybe this is where the red worm of death appears as well.

[Hardy Heinlin]

No, that's sure: the red tape appears before initial flap retraction, at ca. 250 ft RA. But it's not sure whether the height is the real trigger.

[G-CIVA]

With only the books to go by here are the events & triggers that I know of that happen on the take off roll & initial climb out ... there are more I am sure but without the in depth tech knowledge of the pros here I have no doubt I have missed some!

I am unsure what does cause the lower red worm of death to appear, I hope this helps those with a more in depth knowledge find the answer.

Throttle Up - Config Warning(s) Armed, WX Radar Armed, Autobrake 1, 2, 3 , 4 & MAX disarmed, Speedbrake Lever down when Throttle Levers #1 or #3 are out of idle.

Throttle Up + 12 secs - PWS Active (if fitted).

> 8 Kts    Antiskid enabled.
> 20 Kts  Body Gear Steering disarmed.
> 50 Kts  TOGA Arming Ends (On ground).
> 65 Kts  THR HOLD annunciates, Hull Pressurisation Starts.
> 80 Kts  EICAS Cautions Inhibited.
> 85 Kts  Ground Speed - Auto Brake RTO Function Armed - 'Throttle Lever Close' sensor function activates
              when Thrust Levers #2 or #4 are reset below 8 degrees.
> 100kts  PWS Warnings Inhibited

At V1        EICAS Warnings Inhibited.
   
ROTATE - Nosewheel OLEO Extendeds, Stick Shakers Armed, Body Gears Centred.

MLG Lift Off -  Body Gears Tilt -  Air Logic for ALL Systems commences, Windshear Protection Active, Approach Idle Available (If NAI is ON).
   
Initial CLIMB   

50   RA    LNAV Engages, PWS Warnings Active.

400  RA   VNAV Engages, MCP THR Switch Operative, MCP SPD Window Closes if VNAV engages,
              EICAS Warnings become active.

800  RA   EICAS Cautions become active.

1500 RA  Windshear Protection Ends.

[simonijs]

From the KLM AOM, Volume I (Dated 1 JUN 1999), section PFD - Speedscale:
"The red tape indicates the airspeed where stick shaker or low speed buffet occurs.
Not displayed and SPD LIM flag displayed with no computed data or invalid ADC Information."

From Section Flight Controls, Stall Warning:
"When approaching a stall, a signal from either Stall Warning Computer activates stick shakers on both control columns. The Stall Warning System function is enabled when the airspeed is > 140 kts or the pitch is > 5 degrees. Disabling occurs when speed drops below 40 kts."

Stall Warning Computers receive signals from:

Source                        Parameter(s)
AoA Sensors                 AoA
ADCs                           Airspeed and Altitude
IRUs                            Acceleration, pitch and pitch rate
FMCs                           Initial low speed buffet onset
FCUs                           LE and TE flap position
Speedbrake lever         Position
Landing Gear               Position and ground/flight mode

There is one other - maybe relevant - sentence in the (KLM) FCOM: "Nose gear extension sensing provides a signal to relays controlling functions in the stall warning and nose gear steering systems."

Hope this helps.

Kind Regards,
Simon

[Hardy Heinlin]

Thanks for the inspiration, guys.

I've seen two scenarios and the speeds were way above 140 kt, and the heights were ca. 200 and 300 ft.

There is one thing both scenarios have in common, and that one thing makes sense to me:

The time delay from the moment when the pitch passes 5° till the red tape appears is exactly 10 seconds.

I think that's it.


Cheers,

|-|ardy

The Stall Warning System function is enabled when the airspeed is > 140 kts or the pitch is > 5 degrees.

So, once it is "enabled", it becomes "active" after 10 seconds. I think this time delay is not a PFD design feature but just a self-test phase.

[Britjet]

Sounds good to me - I can take some more recordings to see if we can prove it.
Hard to do, though...
I think the main thing is that the stall speed tape isn't there during rotation - which makes sense as you haven't gone flying yet!
Peter.

[Hardy Heinlin]

If my theory is correct: When you stay on the ground and maintain 141 KIAS and a pitch of 6°, the red tape should appear after 10 seconds even if you're not airborne yet.

I see no reason why the stall warning shouldn't also operate on the ground shortly before the aircraft leaves the ground. The stall warning refers to aerodynamics. It's just about pitch and speed. I think the stall warning is also useful when the wheels are touching the ground while the aircraft is "flying", e.g shortly before touchdown or when starting a go-around within 5 seconds after touchdown. Why should the red tape disappear during rotation for a go-around?


Cheers,

|-|ardy

[G-CIVA]

I found two pieces of footage on YT that I hope help a little more ...

https://youtu.be/Zp656c7dwi0

https://youtu.be/WUXdAThygIM

& this piece of a 748F - is it relevant?  Not sure - but the viewpoint is absolutely perfect so I included it anyway ...

https://youtu.be/rfwu--7sQtA?t=5m20s

[Hardy Heinlin]

In all three videos: 10 seconds.

[Will]

If you fail the body gear, load an aft CG, set the parking brake, and tilt the PSX aircraft back on its tail, should the red stall tape appear? I.e., is it only dependent on pitch?

[Hardy Heinlin]

To be precise, I forgot it's OR not AND. >140 kt OR >5°.

However, I'm sure those 5° are not really pitch but AOA. Of course, when the flight path angle is 0°, like on a takeoff roll on a zero-slope runway, pitch and AOA are equal. So the KLM text is not really wrong as long as it refers to the ground (and a flat runway).

Now, when the aircraft is sitting on its tail, the AOA sensor can only give a non-zero signal when a horizontal headwind is blowing against it.

(It could also be that the air data stuff is completely disabled when less than 30 kt are indicated. On the big sims and real decks I always see a 1 knot jump when the airspeed tape stops or starts at the 30 kt limit; it's sort of a switch effect in the 30-31 zone, not a smooth transition like in PSX. I always thought it might be a cosmetic glitch in some big sims.)


|-|ardy

[Jeroen Hoppenbrouwers]

I think the air data stuff including AoA is disabled below 30 knots or maybe even a bit higher. If the AoA sensor is a vane, it will follow gravity, not slipwind, at very low airspeeds.


Hoppie

[Hardy Heinlin]

On the other hand, the EICAS warning inhibit also uses a 5° pitch signal in one of its logical conditions. That comes from the IRS pitch attitude. Both stall warning and EICAS alerts are related to the MAWEA. Maybe the stall warning  does use that same IRS pitch attitude, not the air data AOA, to enable the stall warning during the takeoff roll. So that only the actual stall warning itself uses the AOA. I don't know.

[John H Watson]

On the other hand, the EICAS warning inhibit also uses a 5° pitch signal in one of its logical conditions.... Maybe the stall warning  does use that same IRS pitch attitude, not the air data AOA, to enable the stall warning during the takeoff roll.

Possibly. The Classic 747 used to suffer from nuisance warnings during the takeoff roll. Wind gusts at the gate would flip the AOA vane into unusual attitudes and, at low speeds, the vane couldn't flip itself back to a normal position. During walkarounds, I would make sure that the vanes were not upside down. I don't think these nuisance warnings occur on the 744, so perhaps the logic has been refined with IRU input.

[Hardy Heinlin]

On the other hand of the other hand, the stall warning system then needs to remain enabled once it has been enabled.

It is possible to fly at less than 140 KIAS, and if that is a descent, the pitch may be less than 5°. In that case the stall warning would be disabled. It wouldn't be disabled if the 5° refers to AOA.

[John H Watson]

The Boeing Maintenance Manual says that once the stall warning is enabled, it won't be disabled until the airspeed goes below 40kts. It doesn't mention AOA disabling the stall warning system once enabled.

Of course, there may be videos which disagree with this. The books may be referring to stick shaker and tape or maybe just one of these... and there may be time delays (as seems to be the case on the enabling side)

Ref AMM 27-32-00 p5

[Hardy Heinlin]

10-second delay is now implemented in PSX 10.15:

http://aerowinx.com/board/index.php?topic=4191.0


|-|ardy