SHK calls for changes to ATR-72 envelope

The Swedish Accident Investigation Authority is calling on ATR to limit the maneuvering envelope, or power settings on ATR-72 aircraft after seven severe vibration incidents between 2007 and 2014.

All of the incidents involved the right engine, during descent near 250 knots, after the throttle was reduced to flight idle. In one of the incidents the CVR recorded a 39 hertz sound from an unknown source.

m.aviationweek.com...

Sounds like harmonics. Very dangerous in an air frame. If they know the frequency they should be able to track it down. Of course the 39Hz could be a harmonic of the base frequency but still, not that difficult. I wonder why they cannot find it or ... they have and it is due to something like wing length that is very difficult to do anything about.

Interesting case.

S&F as usual.

P

a reply to: pheonix358

It sounds like a simple harmonic but it's interesting it only happens on the right engine.

originally posted by: Zaphod58
a reply to: pheonix358

It sounds like a simple harmonic but it's interesting it only happens on the right engine.

Yes it does and that in itself points to the blades rotational direction being a major factor.

39Hz has a wavelength of 8.7m or 28ft. You could be looking for half that length or even a quarter but their are limited causes with a length that long.

If it is in the right engine/wing (and assuming clockwise rotation) then the direction of the props will push energy to the surface of the outer wing and to the underside of the inner wing. For the other engine, this energy is reversed.

They are different because the forces are different.

P

a reply to: pheonix358

Great prognosis, . Don't be surprised if you just got robbed.

Kind regards,

bally