Bizzarre airspeed indication

[Pikey]

At 30k feet around 300 IAS by the dial I applied the airbrake. The aircraft pitched up sharply to around 30 degrees but the weird thing was the IAS then began to climb, significantly, by around 30 knots.

 

On decent it further got weird, the IAS descended to zero and pegged there throughout the descent.

 

The IAS indicator recovered after a few minutes at lower altitude.

 

Two questions;

1) Is this something to do with the pitot heat? I used automatic startup and am unsure if the pitot was on.

2) Does that explain the speedbrake effect and how?

 

I'll reproduce if someone thinks this is abnormal, i'm not knowledgeable about aircraft behaviour.

 

Apologies for my lack of knowledge.

[KLR Rico]

An iced pitot can trap air inside the tube, once the air is trapped it's pressure stays the same, but during an ascent this pressure will rise relative to the static air pressure outside the aircraft, this shows up on the gauge as increasing speed.

[lmp]

The IAS problem, as KLR Rico said, looks like pitot icing. As for the aircraft behavior after you opened the speedbrake, maybe you entered a pitch-up?

 

http://en.wikipedia.org/wiki/Pitch-up

[Pikey]

Really interesting about both points, thankyou for your feedback and suggestions. Pitch up is a bit strange because it feels controlled like you added flaps or reset trim - it's interesting about the idea of turbulence over the rear wing caused by speedbrakes, especially if you look at the picture and imagine a high angle of attack. As i said, i've honestly no idea but when I play next i will explore these two phenomena and report back. It's comforting to know this is modelled way beyond my understanding!

[FishBike]

This happens to me on every flight where I stay at 30,000 feet long enough. I'm sure it's meant to be simulating pitot icing, but it happens even with pitot heat on (unless I am not turning it on correctly). It also causes the vertical speed indicator to read 0, which doesn't seem right--that instrument should work based on the static system only. The altimeter, that also works on the static system, continues to function normally.

[Pikey]

I never managed to repro this issue with pitot and noticed pitot isnt on after automatic startup so i do it manually. As for the climb and increase airspeed I believe the climb is just the way the airbrake works and the airspeed was related to pitot.

[Nereid]

Well... how did you manage to get the pitot tube freezed? I have tried to freeze the pitot tube with pitot heat off (just for testing if this is simulated) and even temps below -10 degree but the airspeed indicator still works. Is there some kind of randomness in there or do I have to fly longer then 20 minutes?

[Justin Case]

The pitot always freezes at high altitudes in BST F-86F, even with heating and in clear air conditions...

[Nereid]

The pitot always freezes at high altitudes in BST F-86F, even with heating and in clear air conditions...

 

So I was to low obviously...

[Justin Case]

Yep I think so, try climbing to 30 000 and you should see something happening around 26-27 IIRC. Very annoying.

[Nereid]

Yep I think so, try climbing to 30 000 and you should see something happening around 26-27 IIRC. Very annoying.

 

Yes, confirmed. But its recovering from icing when descending and pitot heat is on (don't know if pitot heat off will make any difference). And the VSI was functioning all the time.

[Corrigan]

I forgot the heating yesterday, and Justin didn't, and I think mine froze before his did. We had the same behaviour on descent, at least, but by then I'd turned pitot heating on.

 

Also, our VSIs did stop working.

[Nereid]

Also, our VSIs did stop working.

 

Well maybe I was not flying long enough or flying in time acceleration differs. My VSI was working all the time. If time allows I will try this without acceleration.

 

But the IAS was behaving as described: ascending and airspeed increase, descending and it goes down. And then it was working again. So yes: Pitot heat is ineffective at higher altitudes.

 

I had an engine failure in my first test. Is this caused by a disabled engine anti-ice?

[Corrigan]

Possibly, I guess. I forgot engine anti-icing too but mine was fine. We need an official response detailing the modelling of these things to be sure, but BST are notoriously bad at replying here.

[Zabuzard]

Had a similiar issue 2 hours ago. (Sadly no track to attach)

 

I flew around 40k feet, all readings were okay (pretty low IAS due to height, mach indicator around 0.7). Then I throttled back, applied speedbrakes and started a descend to ~5k feet. It was no rough descend, pretty smooth.

 

However the speed indications (IAS and MACH) both showed that I'm getting slower (at the end ~0 speed). I was confused, hitted F10 but there I was getting faster like I should. After descend ended + ~2min everything was back to normal again.

 

I'm familiar with how the systems measure the airspeed and pitot heat + anti-ice both where on. Didn't know that a smooth descend from 40k will do that but maybe it is indeed a bug (didn't show up to me for descends ~20k).