PPRuNe Forums - Stick Shakers

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I had a search for this but couldn't find the answer i wanted...

What is the general speed that the stick shaker activates? I read somewhere 5 or 10 kts above stall, or 5% of the CAS.. Or is it dependent in the manufacturer of the stick shaker?

Much appreciated..

It actually will depend on the aircraft design, as that influences the purpose of the stick shaker. If the shaker is purely providing stall warning (the usual function) then the current FAA regulation, § 25.207 - Stall warning states:

Quote:

§ 25.207 Stall warning.

(a) Stall warning with sufficient margin to prevent inadvertent stalling with the flaps and landing gear in any normal position must be clear and distinctive to the pilot in straight and turning flight.

(c) When the speed is reduced at rates not exceeding one knot per second, stall warning must begin, in each normal configuration, at a speed, VSW, exceeding the speed at which the stall is identified in accordance with §25.201(d) by not less than five knots or five percent CAS, whichever is greater. Once initiated, stall warning must continue until the angle of attack is reduced to approximately that at which stall warning began.

(d) In addition to the requirement of paragraph (c) of this section, when the speed is reduced at rates not exceeding one knot per second, in straight flight with engines idling and at the center-of-gravity position specified in §25.103(b)(5), VSW, in each normal configuration, must exceed VSRby not less than three knots or three percent CAS, whichever is greater.

So, as you can see, there's scope for a fair bit of variation, depending on how your stall speed is defined. The very general rule is 5% or 5 knots, but it's only a general rule.

Great, thanks alot for that info!

Some countries require airplanes to be certified with stick "pushers" in addition to stick "shakers." But the stick shaker will always come on before the stick pusher. :ooh:

Quote:

...the stick shaker will always come on before the stick pusher.

Generally true, except...

On one occasion, departing KSBA in an HFB320, the stick pusher activated at 300agl....wham, full forward on the column.
No stick shaker beforehand.
The United California Bank building looked mighty large at the time, especially as we were headed directly for the third story of same...:yuk:
Ahhh, not good.:}

Stick shakers and pushers

The stick shakers originated with the first jets - late 1950s... Some airplanes are so "smooth" when approaching a stall (in "clean" configuration) that they even rarely get activated in such circumstances (i.e. 747)... By chance, 747s do stall very nicely, and fall nose down.
xxx
The stick pusher was a later addition to the stick shakers. The BAC One/Eleven prototype entered in a unrecoverable stall (early 1960s) that the British CAA/ARB required the installation of stick pushers on various "T-tail" airplanes.
xxx
My first encounter with "stick pushers" was in the early Lear 23/24s. We trained for stall recoveries down to stick shaker speed, and never failed to put ignition "ON" for the maneuver, as it was feared that engines might flame-out as well. No need to say, stick shaker/pusher failure was a "NO GO" item.
xxx
:)
Happy contrails

I've had a failure once of the SAS (Stall Avoidance System) in the SWIII Merlin. It has a stick pusher that is controlled by the SAS that receives an input from an angle of attack vane.

Whe were cruising at FL250 with passengers (sleeping) when suddenly the stall warning sounded (very loud horn, like a car horn). Together with that the stick pusher activated and slammed the stick forward, For one second and then the horn and pusher stopped. One second later again the same and it continued like this. Instinctively I cut of the SAS clutch to diconnect the pusher, the stick pusher had activated 3 times by then. Whe could then regain control and take care of the problem.

As we where flying at cruise speed this was a quite scary experience, especially for the passengers.

Bart

Mad (flt)scientist's observations concerning stick shaker trigger margins are quite correct.Stick shakers were introduced because some early jet transport wing sections produced little or no natural aerodynamic buffet prior to the stall and artificial warning became necessary. The stick pusher was introduced on some types, which lacked a natural pitch down at the stall, as a back up to the shaker. On highly swept wing, T-tail jets, where delayed pilot reaction to stick shaker warning could, due to pitch dynamics, let the aircraft enter a high attitude stall from which recovery was not possible, the pusher was essential. This was due to the disturbed airflow from the stalled wing causing loss of airflow over the tailplane and considerably reducing its effectiveness. As previously mentioned, A BAC 1-11 was lost due to this and also Trident G-ARPY during pre-delivery flight test. In the late 60's there we had no true flight simulator for the Trident, so all type rating exercises were flown on the real aircraft.Stall recoveries were always made at the stick shake, with no reliance on the pusher and there was an additional non-standard AoA indicator fitted to help avoid reaching critical pitch angles.