Originally Posted by
bvcu
Static leak at that altitude not really an issue , would only have a small effect , no venturi action on a static , some light a/c have a static source in the cabin for unpressurised types. Cant disagree with the logic of the findings on this system in the report with the information given . Would be of the opinion that a guy of this experience and multiple types and display flying of lighter slower types would be aware of any major altimeter errors visually , so although a possibility don't think its relevant.
I'm not explaining myself as well as I should and re-reading my posts I see that I could have explained myself better.
Consider an aerofoil section of wing. This wing is not hermetically-sealed so the pressure within the wing section will be the sum of the static and dynamic pressures. The dynamic pressure will be dependant on the ram-effect of air entering the leading edge and the venturi effect from the trailing edge.
As an imaginary thought experiment, imagine putting a meteorological barograph into the wing section during flight. Imagine how the needle would fluctuate according to not just altitude but also to true airspeed and angle of attack. Control surfaces might also have an effect albeit this is a fairly simple wing section, the movement of flap and aileron will affect our hypothetical barograph.
The dynamics of this will be in some 1950s yellowing file somewhere, not in an ops manual. Nobody cares or cared, other than the aeronautical engineers who designed the wing section.
Let us now consider the pitot-static head. I suppose it is the traditional two-tube type? On the leading edge of the pitot head there is an orifice. On the trailing edge of the pitot head there is another hole shaped to reduce venturi? I think that's the arrangement on this a/c. The head itself is presumably heatable by electricity? On that day at those altitudes the pitot heating is unlikely to be an issue. So leave the idea of 'freezing' because it seems so unlikely.
From the pitot-static head we have two tubes. One is connected to the forward facing pitot-static head assembly orifice and the other is connected to an orifice on the same assembly in a place where the dynamic pressure expected to be negligible -- i.e. the static vent.
Suppose one of those tubes had a leak. The report states that there had been no recent leak-test. As you said, an a/c being flown aerobatically would be leak-checked regularly if not before every flight. This a/c had not been checked. Furthermore, there is, from the report, a question about the maintenance of one of the components in that system (Alt2 with the wrong serial No.)
In a fast jet being flown aerobatically for performance dynamics might be significant.
These two tubes were so badly damaged in the crash the investigators could not determine whether they had been intact at the time of the final flight of this a/c. This is stated in the report. All they could do was to bench-test the visual display indicators (the instruments). Obviously if the instruments did not get accurate input then they would not have shown an accurate indication. The pilot depends on the indication to fly the a/c.
This possibility does not seem to have been addressed by the AAIB.