VSI indicating descent during rotation

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19th February 2008 | 10:28

#1 (permalink)

Capt. Manuvar

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Joined: Sep 2003

Posts: 286

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From: DNMM/UK

VSI indicating descent during rotation

Why do you get a (approx 500fpm) descent indication during rotation? This has always puzzled me.

19th February 2008 | 10:47

#2 (permalink)

BOAC

Per Ardua ad Astraeus

Joined: Mar 2000

Posts: 18,575

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From: UK

Instrument Pressure Error - research a BA Vanguard crash at Heathrow (a while back) and it will be explained.

19th February 2008 | 11:11

#3 (permalink)

Chris Scott

Joined: Jan 2008

Posts: 2,173

Likes: 51

From: Blighty (Nth. Downs)

October 1965 at LHR. (G-APEB?) Go-around in fog. Highlighted the dangers of excessive reliance on the VSI. The pilot over-rotated, stalling the Vanguard. [The accident also resulted in the concept of an "Approach Ban" if the RVR is below limits, initially 600m for a (Cat. I) ILS.]

On T/O, "ground effect" at rotation also has something to do with the VSI dipping. ILVSIs (instant lapse) and "inertial" VSIs have reduced the problem.

20th February 2008 | 11:51

#4 (permalink)

keithl

Joined: Feb 2003

Posts: 496

Likes: 1

From: Scotland

Just to amplify previous answers a little (may save you having to research), if the static source senses an increase in pressure it will interpret this as a descent. However the pressure increase may have been caused by the change in airflow as the aircraft rotates.

That's as far as I'm gonna go!!