Mode S IAS readout

[nick14]

Hi all.

737-800 mode S reports IAS, I would like to know where this is taken from as in an unreliable airspeed event it could be wrong or very useful.

[ATCast]

In an unreliable airspeed event the Mode S reported IAS should be treated as unreliable too. It comes from the same air data computers.

[Skyjob]

ATCast is correct

[NSEU]

What is unreliable? Captain's airspeed? F/O's airspeed? Standby Airspeed? All three systems have independent sources of pitot and static data. If one significantly disagrees with the other two, then you would assume that the disagreeing one is faulty.

The Captain uses the #1 ADIRU as a source for airspeed, the F/O uses the #2 ADIRU. The left pitot probe feeds the #1 ADIRU. The right pitot probe feeds the #2 ADIRU.

The air data fed to the transponders depends on the ALT SOURCE switch on the ATC panel (assuming all ATC panel models have this switch). If the ALT SOURCE switch on the ATC panel is in "1", the #1 ADIRU supplies the airspeed data to the ATC. If the switch is in "2", the #2 ADIRU supplies the airspeed data.

[Skyjob]

NSEU, not as simple as just that, unfortunately.
Both could be misreading due a number of technical errors.
In which case both ATC 1 and 2 are incorrectly reading.

You CAN however ask for ATC groundspeed to be provided to you.
Maybe ask for wind reports of aircraft at the same level in vicinity.
You can then work out your airspeed from that.
Also remember that your Radio Altimeter is functioning correctly, use it as it may be the only correct source of your vertical situation.

[NSEU]

NSEU, not as simple as just that, unfortunately.
Both could be misreading due a number of technical errors.

Of course, there could be multiple failures. I was just explaining the basics and trying not to overcomplicate things. It's unlikely that both ADIRUs would fail and unlikely that both Captain's and F/O's probes would fail. Probe heat is supplied with power from independent sources (and will even tell you if it has failed).

What does the QRH say?

[Skyjob]

Unfortunately there is no checklist to determine a failed ADIRU.
So no QRH for that.
IAS disagree merely tells you to identify the failed side, IF there is one.
IF one side is found serviceable, it can be used.

Plenty of incidents with unreliable airspeed around, but only few are identical.
K.I.S.S.

[nick14]

Comparing your indications (at least with a 737) has been identified by the manufacturer as incorrect. The FO and Aux probes are on the same side, birdstrike could have affected both at the same speed leaving the reliable one (captains) looking erroneus.

Best way is fly pitch and power and compare with the desired figures from the QRH one in trim and stabilised.

Anyway thanks for the responses.

[NSEU]

Unfortunately there is no checklist to determine a failed ADIRU.
So no QRH for that.

Perhaps because the built-in (continuous) test system will generate flags and remove data on your instruments.

Comparing your indications (at least with a 737) has been identified by the manufacturer as incorrect.

Well, I'm not going to argue with that :O

The FO and Aux probes are on the same side, birdstrike could have affected both at the same speed leaving the reliable one (captains) looking erroneus.

I would expect other symptoms in this case, especially if the bird was big enough to block two pitot tubes :}

[Skyjob]

Perhaps because the built-in (continuous) test system will generate flags and remove data on your instruments.


Having experienced a failed ADIRU due water ingestion I can safely say this is not necessarily the case. The instruments will show various errors but may not be removed from view until the unit output fails for the instrument in question.

Initial indications included IAS DISAGREE, AOA DISAGREE, ALT DISAGREE, A/T LIM, then --- on upper DU, lost predictive information in FMC...
These failures developed into no altimeter tape, no speed tape, on failed side.
All failed indications subsequently recovered 30-45 minutes later into the flight, engineers explained this as the water had evaporated by then as they discovered the signs of water ingestion on the ADIRU affected...
The water affecting the ADIRU was dripping from the Airstairs Drip Tray which had some cracked seals allowing it to sip into the E&E bay below.

[NSEU]

I'm curious to know what were your first thoughts and actions in this situation.

Seeing IAS, ALT and AOA disagree messages on the 737NG might suggest an ADIRU problem (in the absence of probe heat failure indications or birdstrikes). A deeper knowledge of 737NG systems would be required for "A/T LIM" diagnosis.

I'm wondering if drip tray design has improved in the last few years. Their failure has been known to cause multiple computer failures with serious consequences.

[Skyjob]

First thought was fly the aircraft and execute the airspeed unreliable checklist.

Second was at first a guess of a probe problem, as several 737 NG's suffered from unreliable airspeed indications due that in recent events, for iced up probes, but having landed in Mediterranean warm weather and been on ground for ~30 minutes very unlikely. Recently there was even a probe wiring found to be a problem on 737 NG until reversing the wiring sorted the problem. We had been provided background information on these incidents of RYR and NAS both suffering similar issues.
But as AOA was not probe related, we dismissed the probe theory. Altimeter correctness was established using Radio altimeter indicating correctly on operative side and standby instruments.
No checklist for A/T Lim nor when it changed to - - - so maintaining level flight by now using autopilot on functioning side but no auto throttle, a quick reference to FCOM 2 revealed reason and we then concluded from that the possibility of partial ADIRU failure initially as it was the only common source. Later on it failed and recovered afterwards again.
Option to return to field were assessed but as well over maximum landing weight flight continued towards a reduced cruise level and thus destination buying us time and reducing weight en-route. Plenty of destinations en-route to (should need require) divert into.

An airspeed unreliable event does not require a return to nearest alternate, it can be dealt with using pitch and power settings from FCOM-PI-section.
Just remember to declare negative RVSM and set transponder ATC- and ALT-switches to operative side for TCAS.

[Winnerhofer]

Speeds Disagree
Flight crew received no warning that a pitot probe heater had failed.
Stick Shaker Activated
Boeing 737-800. No damage. No injuries.
The 737 was en route with 140 passengers and six crewmembers the evening of Jan. 7, 2012, from East Midlands Airport in England, to Riga Airport in Latvia, where weather conditions were deteriorating. As the aircraft neared the airport, Riga was reporting a 500-ft overcast and a runway visual range of 1,800 ft (550 m) in rain and snow.
Shortly after entering a cloud layer at 6,000 ft during descent, the flight crew noticed a discrepancy in their primary flight display (PFD) indications. The first officer, the pilot flying, saw indications that airspeed was decreasing and that the descent rate was increasing, while the commander saw an indication that airspeed had increased to about 280 kt.
The first officer disengaged the autopilot and autothrottle system, and leveled the aircraft at 4,000 ft. “When the flight crew cross-checked, they noticed ‘IAS DISAGREE’ [and ‘ALT DISAGREE’] warnings on the PFDs and that the first officer’s altimeter read 250 ft higher than the commander’s,” said the report by the Air Accident Investigation Unit of Ireland (AAIU).
While conducting the quick reference handbook (QRH) “IAS Disagree” checklist, the pilots confirmed that pitot heat was selected on and that there were no indications of a pitot heat failure. A cross-check of the standby flight instruments and the inertial reference system showed that the commander’s instruments were providing correct indications.
The commander then took control, and the crew resumed the approach. “Air traffic control volunteered and helpfully gave groundspeed readouts,” the report said.
Shortly after the flaps were extended, the first officer’s stick shaker (stall-warning system) activated. “As the stick shaker continued until the aircraft had landed, the crew commented that they found this very distracting, and the noise made communications difficult,” the report said. “Although … the circuit breaker for the stick shaker could have been pulled, they felt that it was unwise to look for [it] in a dark cockpit and in the prevailing circumstances.”
The crew re-engaged the autopilot and autothrottles, but both systems soon disengaged automatically. The commander continued the approach manually, and after sighting the runway at 300 ft, the crew landed the 737 without further incident.
The report said that a short circuit had developed in the heating element in the pitot probe on the first officer’s side, but the circuit had remained open. Because electrical power continued to be supplied to the heater, no warning of its failure was provided. The consequent ice build-up on the probe caused the air-data discrepancies.
The report noted that 20 similar incidents had been reported by the aircraft operator. “In the majority of the cases reported, the airspeed divergences returned to normal as the probe ice melted during the descent into the warmer temperatures at lower altitudes,” the report said.
Based on these findings, the AAIU recommended that the U.S. Federal Aviation Administration and Boeing review the pitot heating systems in 737NG series aircraft and provide guidance in the QRH about other systems that can be affected and about warnings that can be generated as a consequence of pitot system failure.

[RAT 5]

May I please re-open the debate on the new Boeing B738 QRH for this scenario. I'm teaching this, or trying to, and am very perplexed.

A.N.C was always my creed, and thus was also the philosophy that it is necessary to have a rudimentary knowledge of the various parameters for various phases of flight at various levels. (It is only in recent models that nice amber flags are shown. Previously it was your own brain.) When in doubt level the a/c and apply a suitable amount of thrust; cross check all sources of speed information, and don't fall out of the sky.

The 'IAS disagree' QRH is not memory/recall and it tells you to go to the 'Airspeed Unreliable' checklist which begins with various Memory/Recall actions. (how daft its that sequence). You then proceed with various items and at No.9 you cross check the 3 primary & secondary speed instruments. It could be that the PF has been handling the a/c all this time with the faulty displays. Prior to this, trying to keep the a/c level under ATC instruction e.g. lowish level & 210-250kts you are instructed via QRH to select 4 degrees nose up & 75% N1. This causes the a/c to climb, accelerate and ATC to shout at you. Meanwhile you still haven't conducted the investigation to find out which is the correct display.

Am I missing something here or has immediate A.N.C. and THEN start the investigation been thrown into the trash?

I'd always been taught the QRH is addressed once the a/c is under control; in most cases. Following these latest instructions I wonder if the a/c is really under control at the start of QRH. I feel this change in procedure needs a little more thinking about.

[Skyjob]

The procedure starts by removing any automation from the aircraft:
- Disconnect Auto Pilot
- Disconnect Auto Throttle
- Flight Directors Off

Then advises the crew to achieve control as operative items include: N1% and pitch attitude, so:
- Set pitch and power depending on flaps extension or up

Now the aircraft is in a steady state, subject to weight will go up or down a little but these values are valid for all cross models.

Crew now has time to evaluate the situation and decide on further actions.

AVIATE, navigate, communicate...

To me it sounds Boeing has done a good job at marginalising the error of automation being left in and as per post of Winnerhofer above, determine what the problem is.
In the example above it was a faulty pitot not causing a master caution fault to appear, the reason for this as described earlier was an anomaly and reverse polarisation of the probe mitigates this problem heating the tip first rather then last.
In other cases it could be the ADIRU or another issue altogether.

[nick14]

The new URA checklist is bloody brilliant!

As for the IAS disagree just do it by memory and go straight for memory items for URA.

Set pitch and thrust and tell ATC you have unreliable airspeed and are unable to maintain altitude. Go to the flight with unreliable airspeed tables, set pitch and thrust for phase of flight and WAIT for speed to stabilise. Once the aircraft is level you know you are on speed. Now we cross check the indications but NOT with each other, with the figures quoted in the tables. This is very important as the cross check should always be done with a control figure rather than any number of faulty indications. Hey presto safe flight.

I have tried this in the sim at all edges of the envelope and the memory items are perfect at keeping you safe while you dig out the QRH!