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Be very careful with using the alternate static vent

I didn’t have an accident finding this out, but easily could have had.

A 15kt over-read on the ASI is going to lead to a very low approach speed; much closer to stall.

Administrator
Shoreham EGKA, United Kingdom

Are the errors sufficiently reliable to be corrected?

I mean, could you fly 200’ offf and 15kt fast and it all be ok, or is it unpredictable?

EGKB Biggin Hill

The altitude error is likely altitude dependent. This was at FL120 and +4C (yes +4C).

The speed error is likely to be both altitude+temperature and speed dependent, because the ASI shows the pressure difference.

But I haven’t tested it; will try to get more data points on later flights on this trip (to Greece and back).

Administrator
Shoreham EGKA, United Kingdom

I would guess that the difference in speed will be lower at lower speeds and altitude.

Sweden

The POH should hava data on both altitude and airspeed errors.

ESKC (Uppsala/Sundbro), Sweden

Peter wrote:

A 15kt over-read on the ASI is going to lead to a very low approach speed; much closer to stall.

It may not be 15kt over-read throughout the whole range (e.g. if you’re stopped on the ground, and you pull the alt static, the ASI isn’t going to suddenly read 15 kt). If you have an alternate static, it’s probably worth going out on a nice day and see what your ASI reads all the way from stall speed to normal cruise.

IMHO it’s also a good argument for going up and doing an airwork session every so often, so to keep familiar with how the controls should feel and how the sound of the air going by changes at speeds you don’t normally fly at.

Andreas IOM

There should be an alternate static ASI calibration chart in the POH but it should only be a small difference not 15kts perhaps there is a ‘plumbing’ issue?

Peter I tried the alternate static on my TB20 at 3,000 feet the other day in nearly ISA conditions and there was barely any change in IAS or altitude after an initial jump of both indications lasting well under one second. Pilot-H may have a point.

Rochester EGTO

I once ‘discovered’ a difference of 10kts in a Tomahawk with the alternate open. The angle on climbout looked very wrong so I flew faster than poh speeds, but it could conceivably ended in a spin.

EGCW
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