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View Full Version : Unreliable airspeed. Questions and help needed. (a320)


WhySoTough
20th Sep 2012, 10:26
This was one of the failures that was very very briefly demonstrated to us, and I have a bunch of questions in regards to it.

Firstly, anytime the ECAM identifies the faulty ADR, after ECAM action, everything is back to normal.
If one more ADR is faulty(ADR1+2 for example) I assume still you can use the instruments on the side where it is still operative(this case ADR 3) to come and land at nearest airport.
If on the other hand you can't figure out which ADR is faulty, or you get an ECAM caution of NAV ADR disagree, or have abnormal indications of an increase in speed while climbing, apply memory items immediately, correct?

Then to find the faulty ADR you cross check the 3 ADRs, and maybe use GPS alt and airspeed to match one so you can know it actually is operative.

If it cannot be identified, you use the tables in the QRH for pitch and thrust settings when about FL250, and use the back up speed scale below 250.

Also a question of mine is, how does the standby instrument work exactly? How do we know to consider it faulty or reliable?
With a faulty ADR, even your pressure altitude is unreliable correct? Except the gps alt?

We can use the FPV to maintain straight and level since our IRS are unaffected?

Am I missing something here.
Please correct me or do add any additional info.

Very much appreciated guys, thanks.

Microburst2002
20th Sep 2012, 11:04
This is a failure which hasn't been perfectly addressed in the procedure.

First, I don't have the BUSS, so I can't tell you about it.

The most critical and important part of the whole issue is to come to the conclusion that you don't trust airspeed indications. You don't apply the procedure until then, and you express your lack of trust verbalizing it, as is specified in the SOPs:

"UNRELIABLE SPEED"

then, you have to consider if safety is immediately affected or not. In my opinion, it is always affected, because by the time you realise that you don't trust speed indications anymore, you can be well deep in the sh1t (overspeed, stall...) Remember that you can have these with relatively normal pitch attitudes.

If safety is impacted, then you have to disconnect all automation and then level off (at a safe altitude) but... Is my altitude indication good??? this is not addressed by the procedure. It just tells you to do it, not how. You can use GPS altitude to assess if the altitude indications are reasonable. If you don't have gps, you have to check all three altimeters and decide if they or at least one is healthy, but you have no guidance as to how to decide that. It may be that two statics are frozen and only one is good, so there is mayority of the bad ones...

Once you have levelled off, you can troubleshoot. For troubleshooting it is necessary that you maintain altitude. If you don't, the troubleshooting is totally worthless. Avoid using GS, since wind and TAS will make it differ significantly from IAS. Of course, do not look at TAS. It is compromised and it is not trustworthy. Maintain level flight and use thrust to maintain constant the target pitch attitude in the QRH table. Fixed trayectory and fixed pitch correspond to a given speed (approximately). thrust will vary depending on many factors, specially weight. Maintain path, achieve the target pitch with thrust as required. When settled, use the QRH to find the rogue ADR or ADRs.

then set it, them, OFF and fly with the healthy, or use the tables and fly without IAS indication

KBPsen
20th Sep 2012, 11:24
I would suggest that first of all you learn the basic failure modes of a pitot probe and static port, and combinations thereof, and how they will affect your indications.

Getting bogged down with procedures, ECAM and what not, is not going to help much if you do not understand the basics. The only difficult/challenging part of unreliable airspeed is recognizing it.

Checkboard
20th Sep 2012, 12:19
If on the other hand you can't figure out which ADR is faulty, or you get an ECAM caution of NAV ADR disagree, or have abnormal indications of an increase in speed while climbing, apply memory items immediately, correct?
Yes.

Also a question of mine is, how does the standby instrument work exactly?
Exactly? You want to build an integrated standby instrument system (ISIS)?
It has internal rate gyros for the AH and accelerometers for balance, and an internal Air Data Unit, using the standby pitot pressure and standby static ports. If you have the optional heading display it also uses info from ADIRU 1 (or 3, if switched) for that.
http://www.pprune.org/tech-log/406443-a320-isis-what-adiru-inputs.html

How do we know to consider it faulty or reliable?
It has a Built-in Internal Test Equipment (BITE test) monitor, and you also (of course) cross check it with the left and right displays.

With a faulty ADR, even your pressure altitude is unreliable correct?
It depends on the nature of the failure.

Except the gps alt?
GPS altitude is derived from the GPS, so it's not affected.

We can use the FPV to maintain straight and level since our IRS are unaffected?
No - the Flight Path Vector (FPV) uses barometric information to zero the vertical rate. IF the Baro information is not reliable, the FPV is not reliable.
http://www.pprune.org/tech-log/452973-fpv-fpv-cage.html#post6482965
http://www.pprune.org/tech-log/475005-v-s-geometric-pressure.html#post6974712