A330 VSI in Unreliable Airspeed

[Microburst2002]

Hi.

In the 330, VS indicated in the PFD is not air data, but inertial data, like an IVSI.

May I therefore assume that, in the case of an unreliable speed situation, I can always rely on the indicated VS?

I mean, if the problem is related to the probes, pitot and or static, then the IR part is not affected, and VS should be good, right? That is what I need to know, because the level off for trouble shooting is complicated if you don't rely on your altitude...

However, I am not sure. All those computers interacting with each other... Is the VS a pure intertial data or it has some form of blending with air data?

[A33Zab]

Is the VS a pure intertial data or it has some form of blending with air
data?


VS is a baro-inertial hybrid data (Vzbi).
The inertial signal for dynamic maneuvers and air data for stability in time.

When IR VS is not valid it reverts to baro only, this will be visible on PFD by a amber boxed digital display.

[Microburst2002]

Ok, so I cannot rely on indicated VS in case of unreliable speed because it has some air data in it...

It would be nice to have a push button that made the VS to be purely intertial...

thanks

[Stuck_in_an_ATR]

If the VSI indications made sense and were consistent with altitude, GPS altitude or the FPV, I'd use them. If not, I'd disregard them...

[Lonewolf_50]

Ok, so I cannot rely on indicated VS in case of unreliable speed because it has some air data in it...
Not necessarily so.

Micro, if the VSI is using air data, doesn't that data come from the static ports? (Or an alternate static port if the static ports are clogged? I'd need to look at the A330 system diagram to see how that all lays out).

When UAS crops up, isn't the more common cause trouble with the signal from pitot tubes? That is dynamic air pressure, not static air pressure.

Notional pitot static system. (http://en.wikipedia.org/wiki/Pitot-static_system)

I offer this response with the idea that if your dymanic pressure system is having trouble, the static side of the pitot static system may not be. (Or it might be, any system can have malfunctions).

It is nice that the IR offers the A330 vertical speed input as well.

On the other hand, if your static air data (input) is messed up, that would influence air speed data claculation as well, since airspeed calculation is (if I recall that course from years ago) a comparison of dynamic air pressure to static air pressure corrected for density and temperature.

Which malfunction, or which system malfunction, has brought this question forward?

[Microburst2002]

Well, the thing is that when I have an UAS, I immediately doubt all air data, pitot or static. In such situation we don't have to try to find out what exactly is going on. What we are instructed to do, and I agree with that, is to start the procedure immediately. Troubleshooting will be later.

Therefore, when the procedure tells me to level off... How do I level off if I don't trust the altitude? Then I thought maybe VS would be good enough, but it is air data contaminated, too. I guess GPS alt is the answer, but not all models have GPS fitted.

If the VSI indications made sense and were consistent with altitude, GPS altitude or the FPV, I'd use them. If not, I'd disregard them...

Be careful, we cannot trust FPV in a UAS scenario either. It is also contaminated, apparently...

[Daysleeper]

May I therefore assume that, in the case of an unreliable speed situation, I can always rely on the indicated VS?


No you cannot. Strange things happen within the air data system and it is a heck of a lot more complex than that on a Cherokee. Fly sensible pitch and power and then follow the ECAM and troubleshoot.

[Microburst2002]

Yeah, I'm afraid there is too many circumstances in which you can't rely on anything! Shouldn't be like that.

Complex avionics are awesome, but if they are not detected faulty when they are faulty, they are then deadly.

Currently we have potential deadly situations coming from:

ADRs
AoA
RA

I am becoming paranoid...

[AlphaZuluRomeo]

How do I level off if I don't trust the altitude?
What about pitch & power? :)

[thermostat]

How about ALL the pilot associations/unions demanding the installation of GPS on ALL aircraft in commercial use. GPS is a life saver, no aircraft should be flying without it. So many pilots,crew and passengers would be alive today if they had GPS on board. Time to wake up !!!!

[bubbers44]

I'm sure they had GPS. They just didn't use it correcting for winds, of course. They lost the airplane when the pitot tube iced up.

[Microburst2002]

Quote:
Originally Posted by Microburst2002
How do I level off if I don't trust the altitude?

What about pitch & power?

Pitch and power is just that. You may set a pitch and a power as per the table and you still don´t know if you are flying level or what. That is my problem. The "rogue" ADRs identification depends on how close to the table speed is each ADR, but if you are not flying level, there is not much precision. In my experience, which is a 100% in the sim, where the failures were always pitot, not static related, pilots tend to set the pitch and thrust but they don´t maintain altitude, so the readings are not very meaningful. HOwever, I try to maintain altitude to the feet, and adjust thrust to keep the target pitch, as the procedure asks for, and the good ADR is quickly found.

Actually, the procedure requires altitude keeping.

If I can rely on the altitude, flying the airplane without speed is no problem for me, because I was taught attitude flying.

[Cough]

Yet another case of airbus engineers confusing the issue.

the pilot wants to know:
(a) do I have inertial data or
(b) do I have baro data or
(c) do I have blended inertial / baro data ?

Why not have a 3 position switch above the PFD ?

Ref the pure inertial data. In terms of position, after a few hours we are used to seeing the lateral position drift by a few nm. If you relied on purely inertial v/s this drift would also be significant in the vertical axis -> Soon lead to pilots distrusting the info -> A useless instrument...

Thats why the baro to trim, and the inertial to make it more reponsive...

[A33Zab]

@Ventus:

Exact!

Altitude fluctuations mainly occur due to:


ADM miscalibration or aerodymanic pertubations influencing AoA or static pressure measurements.

local skin deformations
misaligned avionics bay access door
- miscalibration or damaged AoA sensor
- static port obstruction
- ADM hose leakage / Quick disconnect coupling leakage
- Discrepant SFCC discrete input to ADIRU.