VSI indicating descent during rotation

[Capt. Manuvar]

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

[BOAC]

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

[Chris Scott]

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.

[keithl]

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!!

[tristar 500]

October 1965 at LHR. (G-APEB?)

Was actually G-APEE, I unfortunatly heard it but didn`t know next day RIP

Tristar 500