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All About Stick Shakers

Mon Feb 16, 2004 5:28 am

As someone with no experience flying planes, I have a very basic question (I think). I'm curious about stick shakers. I understand that the stick shaker is part of the stall warning system, but that's as much as I know. Can someone describe in more detail what it is--where it is in the cockpit, what it looks like, how it got its name, how it indicates a stall, etc? And to get more technical, what systems does it use to recognize a stall in the first place? Are there any photos where it can be seen prominently? Or is there somewhere online that I can read about it, and other basic cockpit elements? (I searched this forum for stick shaker, and didn't find anything).
Thanks in advance for your help,
Mr Spaceman
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RE: All About Stick Shakers

Mon Feb 16, 2004 5:50 am

Hello Audio19.

You are correct about the stick shaker being part of the stall warning system.

With regards to where it is in the cockpit and what it looks like, well, it's the actual control wheel itself that shakes. So if an airliner is approaching a stall or is already stalling there can be now doubt in a pilot's mind that he's being warned of a stalling condition because the control column / wheel / stick (Airbus), begins to shake right in the pilot's hands.

Regarding the sequence of warnings, I believe a flight crew will receive audible & visual warnings first as their aircraft is approaching a stalled condition, and then the stick shaker begins to shake rapidly back & forth as a stall is starting to happen.

I hope this basic info helps. Big grin

I'm sure the professionals in here will give you more detailed info.

Chris  Smile
"Just a minute while I re-invent myself"
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RE: All About Stick Shakers

Mon Feb 16, 2004 6:16 am

Stick shaking is a function that has been built into airplanes even before the age modern electronic flight deck controls. Airplane wings have a characteristic called washout. This means that the angle of attack (AOA) at the wing root is greater than the AOA near the wing tip. With this arrangement, stall will begin near the root rather than near the tip. The turbulent air coming off the region near wing root will flow over the elevator and cause it to shake. The control yoke will then shake accordingly because of its physical connection to the control surface. While the region of the wing near the root has stalled, the airflow over the majority of the wing is still attached and the pilot has time to recover from the impending stall.

In newer airplanes, particularly those that are fly-by-wire, there is an actual "stick shaker" mechanism that is driven by the flight computer control logic. Controls systems are not my area of specialty so I'll leave it to someone else to elaborate.


[Edited 2004-02-15 22:20:36]
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RE: All About Stick Shakers

Mon Feb 16, 2004 6:39 am

Hi, I hope this helps too:

The stick shaker is part of the stall protection system that provides the flight crew with aural, visual and feel indications of an impending stall.
When approaching a stall condition, the stall protection computer will activate the engines auto-ignition system.
If the angle of attack continues to approach the stall condition limits, the autopilot is automatically disengaged and the stick shaker unit is activated at approximately 1.06 Vs. The stall lights will flash red and warbler sounds.
If the angle of attack reaches a critical point, the stick pusher is activated (at Vs) to move the control column forward. the stick pusher can be selected off at the stall protection panel.

best regards,

That's it! You people have stood in my way long enough. I'm going to clown college!
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RE: All About Stick Shakers

Mon Feb 16, 2004 7:38 am

This can all be summed up by looking at the airworthiness standard concerned. FAA FAR 25.207 states (in part):

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.
[(b) The warning must be furnished either through the inherent aerodynamic qualities of the airplane or by a device that will give clearly distinguishable indications under expected conditions of flight. However, a visual stall warning device that requires the attention of the crew within the cockpit is not acceptable by itself. If a warning device is used, it must provide a warning in each of the airplane configurations prescribed in paragraph (a) of this section at the speed prescribed in paragraphs (c) and (d) of this section.......

So...if the aircraft doesn't provide adequate distinguishable indications (buffet) an alternative warning other than visual must be provided.

This is not a function of the age of the airplane and there is no concrete requirement for a "fly-by-wire" airplane to have a stick shaker. As a matter of fact a fairly new design, the Astra SPX business jet has no stick shaker as the aerodynamic configuration provides good per-stall buffet with plenty of margin.

The stick shaker mechanism itself is an electro-magnetic device that is attached to the control column. It vibrates violently when activiated by the stall protection system.

There are a few different types but a common type is sort of "bell" shaped and about the size of a fist. I have great pictures of a few, that I can't find on my home computer, but I have the next best thing. A link to a company that makes the most of them.


RE: All About Stick Shakers

Mon Feb 16, 2004 7:41 am

A stick shaker is nothing more than a simple vibrator (small electric motor with an offset weight on the shaft) that is attached to the upright column of the control yoke. It simply vibrates the yoke when activated. A stick pusher is tied into the flight control system and "pushes" the nose down when activated. Swept-wing, T-tailed jets can have some definite quirks when it comes to stalls. If an airplane is equipped with both a stick shaker and a stick pusher that is an indication that they really don't want you to get one into a stall. Remember, not all airplanes have stall characteristics that are as benign as a Cherokee.

When it comes to jets, most of the major training facilities (Simuflite and FlightSafety) teach that stall recoveries should be initiated at the first indication of a stall - either airframe buffet or stick shaker activation. Recoveries are initiated by simultaneously rolling wings level, maintaining existing pitch attitude and adding power. Note, you don't lower the nose, you add power. The point is that a stall is not allowed to develop or progress to the point where the stick pusher is activated because the airplanes have a tendency to do nasty things if a full stall develops

Like I mentioned, some airplanes don't really need all of those fancy systems. The Gulfstream G100s that I fly are totally the opposite. They have no stick shakers, pushers and just a rudimentary stall warning system that is disabled 90% of the time! Stall warning is provided by airframe buffet. Stalls are totally benign - a common training maneuver is to pull the power back to idle, hold the yoke in your gut and drive the airplane around holding headings, making turns, etc.

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RE: All About Stick Shakers

Mon Feb 16, 2004 11:17 am

Stick shakers are normaly used on A/C which have servo actuated control surfaces. Due to the one-way characteristic of the system there is no real feel of the forces on the surface transmitted to the pilot, so that these planes have artificial feel as well.
On a directly controled plane the pilot would imideately feel the turbulent air buffeting against the elevator when he approaches a stall. The artificial feel mechanism is usualy just a system of springs, which might have an airspeed input as well, which wouldn´t give him thestall information. So the is a need for a stall warning system, which imitates the action of the airflow ower the elevator.

Je Suis Charlie et je suis Ahmet aussi
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RE: All About Stick Shakers

Mon Feb 16, 2004 12:13 pm

Regarding stalls in smaller aircraft:

Straight wings tend to stall inboard, at the root. You really won't feel the buffet of most light twins and singles in the yolk, you'll feel it in your rear end. Swept wing aircraft stall at the wingtip, not good because you loose roll control due to the lack of smooth airflow over your ailerons.

One of the practical test standards (PTS) for your ratings is to fly the aircraft to a stall and recover, we also do slowflight as Jetguy mentioned, we'll do maneuvers just a couple knots above stall speed to learn the handling of the aircraft at low speeds as they would be on approach.

When doing your primary training you spend quite a bit of time at altitude doing stalls and slow flight so that you recognize it and quickly remedy the situation before it becomes an issue when you are low to the ground and slow.
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RE: All About Stick Shakers

Sun Feb 22, 2004 1:44 pm

Proficiency checks under Part 121 just call for an "approach to a stall" or a minumum steady flight speed. Thus if you get into an actual stall you have busted the parameters for the maneuver. You recover at stall warning.

The A-320 and A-330 don't have stick shakers. No feedback whatever to the stick. They don't so much have stall warning as automatic stall recovery. Separate topic there.

You might be able to glimpse the actual shaker unit in a DC-9 or MD-80 cockpit photo. It hangs on the forward side of each control column a few inches above the floor.

About swept-wing stalls: I once got a terrific piece of instruction. (Thank you Doc, whereever you are.) In a DC-9 simulator the instructor had me trim for hands-off at 10000' and 250 knots, then note the power setting (fuel flow) it took to maintain the speed. Next I sawed the power all the way off and held ten thousand feet even, and let the plane decelerate without trimming. I held it all the way past shaker and until the plane stalled - fully departed controlled flight. Then he said "let go of the yoke and set the power for 250 knots"

I did as he told me and watched. The plane fell pretty much straight ahead and lost nearly two thousand feet or down to about 8000'. It recovered and accelerated then began climbing. It climbed up to about 11700' as the speed began to decay, then fell into a descent again. This one was only down to about 8800 or so, then back up to about 10900 or so. Each successive oscillation was smaller until after only five or six cycles it steadied up on ten thousand feet and 250 knots. Hands off the whole time.

It is called positive dynamic stability. We sometimes forget that it is a design feature of our airplanes.

Now if I'd not been high enough for that first oscillation it would have been a really bad day, but it is nice to know what these planes will do.
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