Didn't want to further clutter the Lion Air thread with more arguments about how pilots/manufacturers should combat unreliable airspeed, but I think it warrants a topic of its own.

Many were suggesting an overly complicated (IMO) mixture of various other sensors using GPS, which leads to problems since groundspeed is very different than IAS (and assumptions on wind cannot be made since a best/worst case scenario could easily put you below stall speed or over VMO/MMO.)

Currently, procedures for (all? most?) airlines have a set pitch and thrust setting you can set initially to keep the plane flying, and then the pilots will reference charts for descent, approach, etc pitch and thrust settings.

I like this honestly, it is simple and effective, but we've had a few crashes causes by this recently (*assuming* the Lion Air crash was related to unreliable airspeed,) so what gives? Is unreliable airspeed not being diagnosed properly? Do the procedures simply not work? Are other automated systems throwing a wrench into the recovery leading to a crash?

I wonder how hard it would be to implement something on the FMCs similar to the one engine out toggle that spits out information (such as max one engine ceiling) and manipulates N1 bugs for situational awareness. You could have some sort of unreliable airspeed toggle that will disable any kind of automatic trimming, could display a thrust setting/pitch attitude for MRC at the current weight/altitude that could bug the displays properly, and put an amber box around what values could be affected by the anomaly (without completely removing them, just in case good data can be taken from it.)

It could also display and/or enlarge the AOA gauge, maybe have expected AOA on the FMC at the targeted pitch/thrust to maybe rule out AOA if it's showing 15 units when it should be 4±2 for example. In addition to charts, the FMC could include prompts for descent/approach/etc that bases values off current weight, altitudes, inputted flap settings, etc.


Just some thoughts, like to hear what the other armchair engineers have to say. I'm not an engineer but I have flown 737s (not for airlines)like foryears, so I'm not completely clueless (though I'm surprised how quickly my brain has dumped 737 knowledge and procedures.)

what do you do as a last resort if you don't trust the figures you are reading? turn 90 degrees so that you are not flying with or against the trade winds to see if values match what you expect based on your experience of flying the type for a period of time?

At my first flying job, we had one airplane that had a pitot heat issue; with the pitot heat on, it would overheat and pop the circuit breaker after 20 minutes ro so. Unfortunately, maintenance couldn't duplicate it on the ground (they also didn't leave it on for 20+ minutes at a time, but that's another story).

I flew that plane quite a bit and became very proficient by flying pitch and power (when the airspeed indication wasn't reliable or even working at all). So that's my thought, that the airline industry needs to reemphasize such a basic concept.

leghorn wrote:

what do you do as a last resort if you don't trust the figures you are reading? turn 90 degrees so that you are not flying with or against the trade winds to see if values match what you expect based on your experience of flying the type for a period of time?

Well, I'd trust the pitch/power settings. They work pretty well.

I don't know if I'd have the wind direction memorized when we went unreliable airspeed so that kinda rules flying perpendicularly (plus, you'd have to turn towards the destination anyway.)

People were talking about all kinds of crazy things with various back up sensors and all. I think pitch/airspeed is pretty much foolproof if you do it correctly, but I'm not against new technology or anything, and currently, we've had some crews mess it up (which might be remidied more training honestly)

I know the 787 and probably the A350 already has a back up presentation for unreliable airspeed


I believe it blends inertial and AOA inputs amongst others to give a reasonably accurate indication

There's BUSS on the Airbus... and if I remember correctly, you can pull out an airspeed calculated by the engine probes on the A350 (but not accurate at low power settings) as a back up to the BUSS and/or pitch-and-power.

Max Q wrote:

I know the 787 and probably the A350 already has a back up presentation for unreliable airspeed


I believe it blends inertial and AOA inputs amongst others to give a reasonably accurate indication

The 787 can display AOA Airspeed and GPS altitude as a backup in case of air data problems.

All Boeing models have a detailed Unreliable Airspeed checklist. The pitch and power settings are memory items.

IMHO the big issue isn't unreliable speed per se. It is awareness that you have unreliable airspeed. If the aircraft systems detect it, automatic switching to backup systems is a straightforward remedy. However if the systems do not detect it, things are much more insidious.

Max Q wrote:

I know the 787 and probably the A350 already has a back up presentation for unreliable airspeed

I believe it blends inertial and AOA inputs amongst others to give a reasonably accurate indication

The A350 does not have Backup Speed Scale (BUSS). It has more advanced automatic switching to backup instrumentation. If all primary airspeed indications have been rejected, it will says "ISIS SPD" for speed sourced from standby instrumentation and "BKUP SPD" for speed sourced from FADEC and AoA.

Newer A330s, and by newer I mean built or retrofitted after around 2007, have BUSS, and A320 series as well I think. It uses raw AoA data to display a "safe speed range" which replaces the speed tape.

mandala499 wrote:

There's BUSS on the Airbus... and if I remember correctly, you can pull out an airspeed calculated by the engine probes on the A350 (but not accurate at low power settings) as a back up to the BUSS and/or pitch-and-power.

It's not so much "pulled out" as automatically used if all three primary sources and ISIS are deemed unreliable.

JT610 did not have unreliable airspeed, the FDR data released shows correct airspeed on both sides.

The 787 and I believe the 777X as well, have a system referred to as Synthetic Airspeed. You can read a little more about it here:

https://www.atsb.gov.au/media/5773029/a ... -final.pdf

Maybe I’m still asleep and need a second cup of coffee before I respond but...... I haven’t been retired THAT long but don’t jets still have standby sirspeed, altitude and horizon? Absolutely we were taught pitch and power settings as most here have said but we also had the standby Instruments that don’t run thru the CADC. Sadly one crew had a CADC failure and in their attempt to trouble shoot never compared the standby instruments to confirm which airspeed was correct. They chose the wrong one, kept getting landing flaps to blow back up and were so fast they ran off the end and flew into Subic Bay.

Starlionblue wrote:

IMHO the big issue isn't unreliable speed per se. It is awareness that you have unreliable airspeed. If the aircraft systems detect it, automatic switching to backup systems is a straightforward remedy. However if the systems do not detect it, things are much more insidious.

Max Q wrote:

I know the 787 and probably the A350 already has a back up presentation for unreliable airspeed

I believe it blends inertial and AOA inputs amongst others to give a reasonably accurate indication

The A350 does not have Backup Speed Scale (BUSS). It has more advanced automatic switching to backup instrumentation. If all primary airspeed indications have been rejected, it will says "ISIS SPD" for speed sourced from standby instrumentation and "BKUP SPD" for speed sourced from FADEC and AoA.

Newer A330s, and by newer I mean built or retrofitted after around 2007, have BUSS, and A320 series as well I think. It uses raw AoA data to display a "safe speed range" which replaces the speed tape.

mandala499 wrote:

There's BUSS on the Airbus... and if I remember correctly, you can pull out an airspeed calculated by the engine probes on the A350 (but not accurate at low power settings) as a back up to the BUSS and/or pitch-and-power.

It's not so much "pulled out" as automatically used if all three primary sources and ISIS are deemed unreliable.

Not just deemed unreliable, but I think the ADIRUs have to be physically switched off right?

Still, it's an amazing system that's clearly well ahead of its time. We have a crash loosely-related to unreliable airspeed, then threads about how airliners should have such technology - yet it's been quietly present on the 330 family for over a decade. Prevention of accidents never make headlines, which is a shame because that's really where some of the most amazing technology and ideas are working their magic. Perhaps not BUSS specifically, but those "pesky" flight envelope protections people like to complain about for sure. If you ever have a minute, look up the "mishaps" and "incidents" that turned out to be a bunch of paperwork / reprimands on AB aircraft over the years. If you connect the dots, some are sure-crashes and match other crash profiles perfectly before protections kick in. Pretty scary stuff.

estorilm wrote:

Starlionblue wrote:

IMHO the big issue isn't unreliable speed per se. It is awareness that you have unreliable airspeed. If the aircraft systems detect it, automatic switching to backup systems is a straightforward remedy. However if the systems do not detect it, things are much more insidious.

Max Q wrote:

I know the 787 and probably the A350 already has a back up presentation for unreliable airspeed

I believe it blends inertial and AOA inputs amongst others to give a reasonably accurate indication

The A350 does not have Backup Speed Scale (BUSS). It has more advanced automatic switching to backup instrumentation. If all primary airspeed indications have been rejected, it will says "ISIS SPD" for speed sourced from standby instrumentation and "BKUP SPD" for speed sourced from FADEC and AoA.

Newer A330s, and by newer I mean built or retrofitted after around 2007, have BUSS, and A320 series as well I think. It uses raw AoA data to display a "safe speed range" which replaces the speed tape.

mandala499 wrote:

There's BUSS on the Airbus... and if I remember correctly, you can pull out an airspeed calculated by the engine probes on the A350 (but not accurate at low power settings) as a back up to the BUSS and/or pitch-and-power.

It's not so much "pulled out" as automatically used if all three primary sources and ISIS are deemed unreliable.

Not just deemed unreliable, but I think the ADIRUs have to be physically switched off right?

Still, it's an amazing system that's clearly well ahead of its time. We have a crash loosely-related to unreliable airspeed, then threads about how airliners should have such technology - yet it's been quietly present on the 330 family for over a decade. Prevention of accidents never make headlines, which is a shame because that's really where some of the most amazing technology and ideas are working their magic. Perhaps not BUSS specifically, but those "pesky" flight envelope protections people like to complain about for sure. If you ever have a minute, look up the "mishaps" and "incidents" that turned out to be a bunch of paperwork / reprimands on AB aircraft over the years. If you connect the dots, some are sure-crashes and match other crash profiles perfectly before protections kick in. Pretty scary stuff.

On the A350 the ADRs (not the entire ADIRUs) don't necessarily have to be switched off to get Backup Speed display. If the PRIMs and ADIRS reject all three ADRs and the ISIS air data, Backup Speed (based on FADEC and AoA vanes) is automatically displayed.

The crew can indeed turn off all three ADRs manually if unreliable airspeed is suspected/concluded, but ISIS speed would still be used unless that was rejected by the PRIMs. In this scenario, the PRIMs would use the backup source to monitor ISIS.

Starlionblue wrote:

estorilm wrote:

Starlionblue wrote:

IMHO the big issue isn't unreliable speed per se. It is awareness that you have unreliable airspeed. If the aircraft systems detect it, automatic switching to backup systems is a straightforward remedy. However if the systems do not detect it, things are much more insidious.



The A350 does not have Backup Speed Scale (BUSS). It has more advanced automatic switching to backup instrumentation. If all primary airspeed indications have been rejected, it will says "ISIS SPD" for speed sourced from standby instrumentation and "BKUP SPD" for speed sourced from FADEC and AoA.

Newer A330s, and by newer I mean built or retrofitted after around 2007, have BUSS, and A320 series as well I think. It uses raw AoA data to display a "safe speed range" which replaces the speed tape.



It's not so much "pulled out" as automatically used if all three primary sources and ISIS are deemed unreliable.

Not just deemed unreliable, but I think the ADIRUs have to be physically switched off right?

Still, it's an amazing system that's clearly well ahead of its time. We have a crash loosely-related to unreliable airspeed, then threads about how airliners should have such technology - yet it's been quietly present on the 330 family for over a decade. Prevention of accidents never make headlines, which is a shame because that's really where some of the most amazing technology and ideas are working their magic. Perhaps not BUSS specifically, but those "pesky" flight envelope protections people like to complain about for sure. If you ever have a minute, look up the "mishaps" and "incidents" that turned out to be a bunch of paperwork / reprimands on AB aircraft over the years. If you connect the dots, some are sure-crashes and match other crash profiles perfectly before protections kick in. Pretty scary stuff.

On the A350 the ADRs (not the entire ADIRUs) don't necessarily have to be switched off to get Backup Speed display. If the PRIMs and ADIRS reject all three ADRs and the ISIS air data, Backup Speed (based on FADEC and AoA vanes) is automatically displayed.

The crew can indeed turn off all three ADRs manually if unreliable airspeed is suspected/concluded, but ISIS speed would still be used unless that was rejected by the PRIMs. In this scenario, the PRIMs would use the backup source to monitor ISIS.

Thanks for the info - I'm loosely familiar with the "conventional" FBW systems, but the 350 is a different animal, and what you say makes perfect sense (and makes an already good system even better/safer). On the 330s though I think they have to be manually switched off, correct?

The level of redundancy on the latest generation aircraft is really incredible. Especially when you factor in hydraulic failure modes and the electronic backups.

estorilm wrote:

Starlionblue wrote:

estorilm wrote:

Not just deemed unreliable, but I think the ADIRUs have to be physically switched off right?

Still, it's an amazing system that's clearly well ahead of its time. We have a crash loosely-related to unreliable airspeed, then threads about how airliners should have such technology - yet it's been quietly present on the 330 family for over a decade. Prevention of accidents never make headlines, which is a shame because that's really where some of the most amazing technology and ideas are working their magic. Perhaps not BUSS specifically, but those "pesky" flight envelope protections people like to complain about for sure. If you ever have a minute, look up the "mishaps" and "incidents" that turned out to be a bunch of paperwork / reprimands on AB aircraft over the years. If you connect the dots, some are sure-crashes and match other crash profiles perfectly before protections kick in. Pretty scary stuff.

On the A350 the ADRs (not the entire ADIRUs) don't necessarily have to be switched off to get Backup Speed display. If the PRIMs and ADIRS reject all three ADRs and the ISIS air data, Backup Speed (based on FADEC and AoA vanes) is automatically displayed.

The crew can indeed turn off all three ADRs manually if unreliable airspeed is suspected/concluded, but ISIS speed would still be used unless that was rejected by the PRIMs. In this scenario, the PRIMs would use the backup source to monitor ISIS.

Thanks for the info - I'm loosely familiar with the "conventional" FBW systems, but the 350 is a different animal, and what you say makes perfect sense (and makes an already good system even better/safer). On the 330s though I think they have to be manually switched off, correct?

The level of redundancy on the latest generation aircraft is really incredible. Especially when you factor in hydraulic failure modes and the electronic backups.

The A330 is a bit less sophisticated, as it doesn't have ISIS SPD and BKUP SPD options on the PFD. Turning off all three ADRs isn't in the unreliable airspeed procedure, because, I think, it wouldn't really do anything useful. It has BUSS on the newer builds. If you want to use ISIS speed, you can do it directly from the ISIS display, as of course, you can also do on the A350. ADR3 and ISIS use the same probes, while ISIS has its own on the A350. AFAIK you can't get speed information from FADEC on the instruments.

Your first flying lessons were supposed to highlight that ATTITUDE + POWER = PERFORMANCE. Reliving my early days, that exercise of Attitudes and Movements was glossed over so quickly as to be dismissed.

But it works. And that is the most fundamental theory of flight. And the basis of Flight With Unreliable Airspeed charts in all large jet airliners.

The Key to a successful use of this procedure in a large jet airliner? DON'T PANIC WHEN YOU HEAR THE STALL WARNING TOGETHER WITH THE OVERSPEED AT THE SAME TIME.

You're not going to die. Unless you panic.

Yikes! wrote:

Your first flying lessons were supposed to highlight that ATTITUDE + POWER = PERFORMANCE. Reliving my early days, that exercise of Attitudes and Movements was glossed over so quickly as to be dismissed.

But it works. And that is the most fundamental theory of flight. And the basis of Flight With Unreliable Airspeed charts in all large jet airliners.

The Key to a successful use of this procedure in a large jet airliner? DON'T PANIC WHEN YOU HEAR THE STALL WARNING TOGETHER WITH THE OVERSPEED AT THE SAME TIME.

You're not going to die. Unless you panic.

All of the Boeing Unreliable Airspeed checklists have pitch and power settings that are memory items. First steps are to disconnect the automation and fly pitch and power as stated in the procedure.

Out of curiosity from a no -Flyer, are those ‘unreliable airspeed’ charts fairly accurate? I’ve seen a couple here and there.
If it says at a certain weight and max climb thrust (always seems to be at 290kts), you’d achieve a 1600ft/min climb rate, is that reasonably accurate? Perhaps give or take a couple hundred feet per minute either side of that? Likewise with the descent portion?

The "Unreliable Airspeed" drill is pretty simple, but as stated above, you have to know the problem exists.

But ... the issue is not ongoing trouble shooting at cruise, the issue is getting the aircraft away from the ground safely. That is why you have differrent nose attitudes/thrust settings varying from take-off to thrust reduction altitude, then from there to 10,000', then higher. Once you are clear of any immediate threat, then and only then, can you consider your options and other sources of information.

Historically, the aircraft that have been lost due to Unreliable Airspeed have been close to the ground. Otherwise, the successful occurances we have never heard about. Namely, they were either handled properly, or occurred at a higher altitude.

Maybe I am a dinosaur, but I have always like the stand by instruments on older Airbuses. Airspeed, attitude and altimeter that look like someone stole them right out of a Cessna 172. Basic, easy to read and not attached to anything other than pitot/static pressure!

I'm familiar with a large transport airplane that very recently was without correct flight data, had failing data, and lost the standby instruments, at night, in a very mountainous region. The crew flew pitch and power and despite conflicting information, determined the incorrect data, climbed above IIMC and made a return on raw data.

Pitch and power is appropriate, and should be monitored all the time, particularly on approaches. Even with everything working, a knowledge of correct pitch and power will reveal an incorrect weight or approach data such as speeds, by showing a different power requirement, or pitch. Crew should be monitoring pitch, power airspeed, and other parameters all the time, instead of blindly following a flight director or other automation cues.

Aircraft today have groundspeed information, which can be used to approximate aircraft speed, as well as angle of attack information, pitch limit indications, and other means of determining how much angle of attack remains for a given pitch and power combination, all of which can be used to fly the aircraft, to say nothing of standby instrumentation.