# Why do (some) aircraft shake at low speeds with flaps, slats extended?

I landed a couple of times as a passenger with the Embraer 190 (I like Embraer very much) on runways with a length below 2 km.

I noticed that the plane started to shake or vibrate quite strongly once reducing its speed prior to landing. The flaps during this time were fully extended. How can one explain these vibrations? I have to note though, that I often sit in the back of the fuselage, where vibrations tend to be more pronounced.

• I would say that the shaking started about 2-4 minutes before touchdown. Just from my memory, I didn't measure the time. Oct 30 '16 at 12:27
• It would be helpfull if you find a video showing those vibrations. This kind of phenomena is hard to describe otherwise Oct 31 '16 at 8:51
• thanks for all the answers! Yersterday I landed again as a passenger with the Embraer 190 on a runway with a length of 2500 meters and didn't notice these vibrations. I posted the question after landing on London City Airport, whose runway length is 1500 meters. I agree that load or weight of the aircraft is an important factor. Everything else being equal, a heavy aircraft will have a higher angle of attack. Up to now, based on your questions, I think that the disturbed airflow hitting the horizontal stabilizer is the answer. Thanks again. There is a lot to learn here. Nov 5 '16 at 18:18
• I landed again on London City Airport as a passenger with the Embraer 190 and it seems that the steeper flight path is responsible for the shaking. I assume that the steeper flight path leads to higher angle of attack, causing the horizontal stabilizer to be exposed to "dirty air" or turbulences induces by the wings, flaps. Nov 12 '16 at 13:55

There are many phenomena which can cause vibrations. Without knowing further details like the frequency, starting moment or the weather conditions on landing it is difficult say. Common causes are:

Stall buffet/ self induced turbulence Normally when a plane lands it flies with low velocity and a high lift coefficient, near to stall. The flow detaches much earlier and there is more turbulence behind the main wing. The tailplane has to fly through this turbulent zone and hence you feel the vibrations. Sometimes deploying the spoilers, even if you're not flying at stall speed is enough to create turbulence which will hit the tailplane. T-tail airplanes are less affected because they're further away from the air zone influenced by the main wing.

Atmospheric turbulence Can be created due to thermals (in that case it should be sunny) or wind hitting ground structures. If you have landed a couple of times and always felt the vibration it probably isn't the answer.

Wake turbulence Shouldn't be your case either, separation minima exist to avoid this problem and it would probably feel different every time you land.

• @mins Possibly, I hadn't seen the time comment. Perhaps someone who has flown the E190 can provide a better answer. Oct 30 '16 at 12:43
• I doubt stall buffet is the answer here. Approach speed (Vref) is no less than 1.3 times stall speed in the landing configuration, and 2-4 minutes out the speed flown is probably at least Vref+20. Vibration from gear would seem more likely here. Oct 30 '16 at 13:55
• I agree with Jonathan that stall buffet is unlikely, but I suspect the added turbulence from the flaps and the high AoA affecting the horiz stab is probably the key. Especially since he was in the rear Oct 30 '16 at 19:32
• You may also mention wake turbulence. During landing each aircraft folow the exact same final approach route to a given runway Oct 31 '16 at 8:56
• Wake turbulence, mentioned by @ManuH, should not be an issue, at least not in routine operations. Air traffic control separates airplanes based on wake turbulence characteristics so that there should be no effects on following airplanes. It's possible, especially when operating not under ATC control, to be in too close, but you probably won't be feeling shaking: you'll feel more like a rolling motion. Nov 1 '16 at 2:55

The answer is quite simple: Altitude. Air near the ground, as a general rule, is more turbulent and unpredictable than air aloft. Buildings create mechanical turbulence and there are pockets of updrafts from surface heating (especially over black asphalt runways).

You may also be feeling mechanical vibration from the gear extending, or maneuvering causing airframe oscillations.

Flaps and slats increase the surface area of the wing, which reduces wing-loading, which reduces wake turbulence. The aircraft creating turbulence for its own tail is likely not the answer, and in extreme situations this is actually referred to as a "deep stall" which is quite dangerous.

There are many reasons why the plane is shaking.

1. The plane may be experiencing turbulence, wind causing plane to not balance.

2. The plane may stall because the angle of attack is too great.

3. The engine may have problems running at slow slow speeds

• You should do some editing effort (such as line break). An answer is easier to read when well formatted. Oct 31 '16 at 8:54
• @ManuH: Daniel did format it correctly, but didn't know about adding two spaces (or a blank line or <br>) to force a line change.
– mins
Oct 31 '16 at 18:43