Wizz three separate unreliable airspeed incidents in same day departing from SOF

[andrasz]

In a highly unusual sequence of events WizzAir had three separate unreliable airspeed incidents out of Sofia yesterday, all on A321. Two of them diverded to BUD due to WX at SOF, the third rejected takeoff:
Incident: Wizz A321 at Sofia on Feb 26th 2018, unreliable airspeed (http://avherald.com/h?article=4b5728f2&opt=0)
Incident: Wizz A321 at Sofia on Feb 26th 2018, unreliable airspeed indications (http://avherald.com/h?article=4b572ccf&opt=0)
Incident: Wizz A321 at Sofia on Feb 26th 2018, rejected takeoff due to unreliable airspeed (http://avherald.com/h?article=4b572e7b&opt=0)

[fox niner]

Either Wizzair or Airbus need to come up with a damn good and fancy explanation, or they are in for a rough ride. You can’t have this happening and pretend it didn’t happen.

[andrasz]

An interesting comment on avherald, worth repeating here:
year ago same/similar situation happen in MXP (milan). Three different aircraft (MD80) with same simptom (pressing TO/GA push button pitch FD bar goes out of wiew on PFD's) While try to find out what's going wrong on the first two aircraft a therd aircraft called for same issue. We realized that the three aircraft was parcked with same "heading" and find out static port (MD80 have a pneumatic pipe line connection LH/RH static port's)pipe line blocked due ice.Very low temperature (high unidity) during night and wind blowing ortogonally respect the aircrafts position. Others same aircraft type parcked in different heading respect the three affected did not report issue

[Mäx Reverse]

This is a known issue on the A320 family, especially in those weather conditions that were prevailing on the respective night in SOF: moderate to heavy snow all night long with significant sub-zero temperatures.

During cockpit preparation the PROBE/WINDOW HEAT p/b is switched from AUTO to ON according FCOM PRO-NOR-SUP-ADVWXR. This is in order to keep the windshields and windows clear. However, if any snow on the windows is not removed prior to switching on the window heat, the snow will melt and the draining water might refreeze on the cold fuselage. Unfortunately the path of the melting water leads to ice ridges directly in front of the pitot tubes.

Therefore there are two cautions in the FCOM. One is located directly before the aforementioned action line in the Cockpit Preparation:

CAUTION: With ice or snow accumulated on the windshield and/or the upper cockpit fuselage, and if the PROBE/WINDOW HEAT is on, melted ice or snow running down from these areas could re-freeze on the fuselage area below, if the temperature is very low. This could create ice build-up on the forward fuselage that could possibly disturb the airflow around the static/pitot/angle-of-attack probes.

Another caution is located in the De-Icing Procedure unter UPON COMPLETION OF THE SPRAYING OPERATION below the action line PITOTS and STATICS (ground crew) ... CHECK:

CAUTION: When the OAT is low (below -5 °C) during snow/freezing rain precipitations, melted snow or raindrops may drip from the cockpit windshields and freeze on the fuselage below. This could create ice build up on the forward fuselage that could possibly disturb the airflow around the static/pitot/angle-of-attack probes, and result in unreliable air data measurements during takeoff. Therefore, during taxi out before takeoff, beware of this possible build up of ice. The area around static/pitot/angle-of-attack probes must be free of ice/snow before starting takeoff.

So it is essential that the snow from the windows and upper fuselage above the flight deck is removed and that the area in front of the pitot probes is checked if conditions are so that existing melting water might refreeze.

Otherwise it will look like on the picture below...

[ATC Watcher]

Council van : Did ATC have to tell them to climb above the MSA?
If they were on a heading after departure, yes it is ATC responsibility to keep them clear of terrain..
The transition altitude of 12000ft is a bit of a clue?
Nothing really to do with it.

[EDLB]

Hope that there’s is a thorough investigation about the root cause. That first hole in the cheese is way to dangerous if not corrected.

[andrasz]

@Mäx, many thanks for the detailed explanation, this well could be the case. This also explains why they decided to continue to BUD, I would assume the problem is the worst at T/O and progressively clears as the airstream gradually eliminates the ice.

[bvcu]

MAX , logical description , however in this day and age would you have somebody on a headset with enough training to know what to look for . Lucky enough to work for a wide body operator that insists on an engineer/technician on the headset every departure but know thats not reality for most these days

[ATC Watcher]

Council Van : Transition altitude is not constant for the whole of Bulgaria and does vary from airfield to airfield,
Did not know. Thanks for the info. In many places the TL is uniform and really not relevant to the terrain in most areas in the country. Just take USA as example.

For not trusting the controller with terrain, you have nowadays to trust the system,i.e. follow the SIDs and the vectors and alt restrictions given by ATC on departure, and it is ATC responsibility to keep you clear from terrain.( ICAO PANS OPS) Under your own navigation it is your responsibility, but fortunately many ATC systems have today a terrain warning based on mode C/S returns that will warn the controller in case you are too low to penetrate certain areas.
Anyway in this case here, if we are to believe the AvHerald, the controller spotted the problem.
The decision by the crew to stop climb at 10.000 was probably taken during the stress of the event , and, when told, ATC intervened and added safety .
The " communicate " part did help here.
The whole interaction seemed to have worked quite well I would say.

[LookingForAJob]

For not trusting the controller with terrain, you have nowadays to trust the system,i.e. follow the SIDs and the vectors and alt restrictions given by ATC on departure, and it is ATC responsibility to keep you clear from terrain.( ICAO PANS OPS)A bit of thread drift but it's been a topic of discussion - with no clear answer - in the past, but is it the controller who is responsible for terrain clearance when an aircraft follows a SID? It's arguable that the the pilot is on his/her own navigation.

On the main topic, if Mäx's suggestion is anywhere close to a contributory cause, there are a lot of questions that need to be asked about how crew operating these aircraft either are unaware of or not checking this 'gotcha'.

[PEI_3721]

Presumably the left and right pitot-static locations are similar, thus equally likely to have a problem.
Is the standby system positioned in a significantly different location, thus less likely to simultaneously suffer the same problem?

Even with a viable third system the crew would have to decide which system(s) were more reliable; normally an airdata comparator would flag a discrepancy between the primary systems requiring the crew to choose the best (matching) ‘two out of three’ systems. However this is a much more serious problem if two systems are unreliable.
Such a situation could defeat most 2/3 comparators; the technology is deemed to have failed, thus safe flight depends on human skill - a situation which technology is unable to resolve, yet where technology is used because the human is similarly unable to choose with certainty. ( AF447, and previous A330 events - airdata, Sweden CRJ - attitude).

[EGPFlyer]

A bit of thread drift but it's been a topic of discussion - with no clear answer - in the past, but is it the controller who is responsible for terrain clearance when an aircraft follows a SID? It's arguable that the the pilot is on his/her own navigation.


On a SID you are only assured terrain clearance if you meet the relevant altitude constraints. Only the pilots are resonsible for that.

[eckhard]

yes it is ATC responsibility to keep them clear of terrain

I would agree with some of the previous posts in that the pilots are ultimately responsible.
e.g. the recent incident at Medford, OR?

I suppose it’s a bit like, “cleared to land/take-off”. The ATC guy/gal gives the clearance but the pilots still decide whether or not it is safe to do so.

[tubby linton]

I was quite surprised recently to be told that there were bits of the airframe that could not be treated with anything but hot water, the area around the cockpit being one of them. The problem was that blowing snow had settled and frozen in this area and the only treatment was hot water to melt it, I had suggested using a brush to try and remove it. My concern was that having washed and melted it it off how quickly would the now damp area cool and freeze. No holdover time exists just for hot water and a visual inspection had to suffice.

[Patanom]

I think, this is the sideslip.

[pilot9249]

On a SID you are only assured terrain clearance if you meet the relevant altitude constraints. Only the pilots are resonsible for that.

Are we really, actually debating whether or not ATC has greater accountability for terrain clearance than the flight crew?

Really? Actually?

[DaveReidUK]

I think, this is the sideslip.

Interesting, but I'm struggling to see the relevance to icebound pitot/static/AoA probes.

[buzzc152]

Contaminated deice fluid.

[ATC Watcher]

@sptraveller re we really, actually debating whether or not ATC has greater accountability for terrain clearance than the flight crew?
I think some mix up " responsibilities" with "decision making" and now with " accountability "
But this is not for here, if you want to debate this , open another thread and I'd be more than happy to provide you the facts and the PANS OPS ATM references.

[LookingForAJob]

^ +1

No-one with any sense will suggest that anyone but the aircraft commander is ultimately responsible for many things about a flight, including terrain clearance. But this is more about understanding what is assured by SOPs and the responsibilities of each party in different situations. Maybe if these points are not clear on the flight deck a separate thread might be useful.

[RAT 5]

What about the causes that motivated the thread opening? Any more known?

Remember the Air XL A320 that was being flight tested in Spain by Air NZ crews and crashed during stall testing due to frozen sensors. Ok, it was after power washing and water froze in a hidden area. It was unforeseen; but then how many accidents/incidents are caused by unforeseen matters? Still quite a few.
It's one of those issues with sensors & computers. Even in cars, a faulty sensor in a catalytic burner can cause the engine not to start. Things in FBW flight controls are more critical. How possible is it to by-pass such inhibiting sensors; if you can even trouble shoot to the root cause?

[The Ancient Geek]

The Air XL A320 crash was caused by pilot error, nay gross stupidity.
The test schedule was VERY clear that the AOA sensor check MUST be conducted at or above 10,000 feet AGL. The damn fool (not a qualified test pilot) conducted the test at low level where the result of a failure was fatally predictable.
Every test can either pass or fail, that is why they are on the test schedule. The purpose of the test was to check that the AOA sensors were working correctly - the test failed.
DUH.

[Patanom]

Interesting, but I'm struggling to see the relevance to icebound pitot/static/AoA probes.

How can ice appear on the sensors? There is full automation of the heating activation.
I believe that it is necessary to monitor the pressure sensors of statics.A device can be to connect in parallel
sensors (this is 0-5 volts, LPF) devices from both sides, write everything to a computer, and then conduct an analysis.
Suppose we carry out such a study on about 100 aircraft.

[gums]

Remember, the contamination may not even be frozen!

The USAF B-2 loss in Guam was simply water/moisture in the system - no ice involved.

[DaveReidUK]

Patanom

OK.

I was rather hoping you might explain your strange reference to "sideslip".

[Patanom]

Patanom

OK.

I was rather hoping you might explain your strange reference to "sideslip".
Sorry, I probably misused the term "sideslip". The static pressure sensor is more susceptible to the shadow from the fuselage (the airplane is flying along the COG). The measurements will give, I hope, a more complete picture of the static pressure on the right and left.

[tubby linton]

Does anybody have a copy of the Airbus Iceman booklet or the applicable parts of the AMM they could send me please?