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Using GPS ground speed to resolve Unreliable Airspeed

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Using GPS ground speed to resolve Unreliable Airspeed

Old 2nd Jun 2019, 08:47
  #81 (permalink)  
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STEP 4 737 QRH UAS
4 CrosschecktheIRSandFMCgroundspeedand winds to determine airspeed accuracy if indicated airspeed is questionable.
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Old 2nd Jun 2019, 09:03
  #82 (permalink)  
 
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Originally Posted by yanrair View Post
YOU DONT KNOW THIS unless you notice your ground speed falling below where it was, or lower than flight plan.
essentially this is the very point.
The flight plan GS is the average for a leg which might be over an hour long. Plenty of times Iíve seen the GS change by 40 or 50 kts in a couple of minutes, due to wind changes - which means GS is ok as a rough confirmation, but remains a fairly blunt instrument. Which is probably why the A330 procedure is AoA-based, and doesnít refer to GS at all.
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Old 2nd Jun 2019, 09:11
  #83 (permalink)  
 
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Speed can start falling if power say 2% too low say., due to ISA + 20 for example.
Nose needs to go up. Speed falls more YOU DONT KNOW THIS unless you notice your ground speed falling below where it was,
If the nose needs to go up - then that is the indication you are flying too slowly!
You don't need ground speed to confirm that.
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Old 2nd Jun 2019, 09:26
  #84 (permalink)  
 
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Originally Posted by ShyTorque View Post
What's wrong with the tried and tested method of: "The correct attitude + correct power setting = correct speed?"

Having said that, at very low speeds landing away from an airfield (helicopter) I monitor the GPS groundspeed against the IAS to determine/confirm into wind or downwind on the approach.
Fully agree. That are the procedures made by the producer of the aircraft. It does not state "pull your phone and check the GPS speed for X-reference". Why? Well, every commerical pilot should be able to answer this question for himself. If it goes Ultralight/Light, why not. This things go so slow, huge stall margins, no harm to use a GPS GS there. But at airliner level it seems very unprofessionel - "children of magenta" springs to mind here. After flying "raw data" with FMC it may make perfect sense for some to use a GPS GS to figure out WHICH IAS indicator is the working one(and this is the goal of pitch/power values x-referenced for altitude, weight, configuration).

My advise, if it comes to that in real life - follow published procedures. It will keep your a** safe if you make it throught the "event"(which you should IF you follow unreliable airspeed procedures, they work like a charm(at least in Boeings and old Airbus(A300, do not know the new ones)).
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Old 2nd Jun 2019, 16:52
  #85 (permalink)  
 
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A bunch of AFCS systems revert to GS on approach in case of turbulence, even the bus. It's a fine reference for a minimum speed and the maths for wind correction isn't taxing. If you have nothing else then referring to GPS speed is an excellent idea and I've read it in more than one FCOM.

Here is what is says in my current QRH unreliable airspeed after pitch power initial items:
d) Check all available data sources, including:

ē FMS, POS - GNSS for ground speed,
ē FMS, POS - GNSS INFORMATION for GNSS ALT (if required),
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Old 2nd Jun 2019, 19:02
  #86 (permalink)  
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Originally Posted by Goldenrivett View Post
If the nose needs to go up - then that is the indication you are flying too slowly!
You don't need ground speed to confirm that.
You are quite right but since 1 degree pitch change = about 7 knots - and don't forget you are hand flying so its not easy to judge pitch to within 1 degree, GS does is for you. Why rely on pitch and power only when you have a friend for free?
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Old 2nd Jun 2019, 19:10
  #87 (permalink)  
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Originally Posted by itsnotthatbloodyhard View Post


The flight plan GS is the average for a leg which might be over an hour long. Plenty of times Iíve seen the GS change by 40 or 50 kts in a couple of minutes, due to wind changes - which means GS is ok as a rough confirmation, but remains a fairly blunt instrument. Which is probably why the A330 procedure is AoA-based, and doesnít refer to GS at all.
That is true on very long oceanic flights but we are talking generally here, and in any event, AF 447 went from safe light to chaos in a matter of a couple of minutes where is they had maintained steady state and GS that would not have happened.
And in the majority of cases you do know your wind speed very accurately. A lot of UAS incidents occur at low level and wind is known exactly. And it is at low level, AF 447 apart, that the problems usually need quick resolution. And GS does it for you every time.
Y
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Old 2nd Jun 2019, 19:15
  #88 (permalink)  
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Originally Posted by pineteam View Post
Yanrair, if it was a good idea to use GPS in an unreliable airspeed case, aircraft manufacturers would recommend using it.
Read your previous post again: 2 serious mistakes that in real life will put you in a dangerous situation. Donít try to reinvent the wheel.
Dear Pineteam
They do. Another thread shows Airbus agrees with Boeing

Boeing STEP 4 737 QRH UAS
4 Crosscheck the IRS and FMCgroundspeed and winds to determine airspeed accuracy if indicated airspeed is questionable.

And the fact that I made an error earlier in a calculation has no bearing on the validity or otherwise of the arguments being put here. My error was made while lying in bed typing and trying to imagine the situation I was describing. In a real plane that would obviously not occur. Either GS is a good tool or it is not. It is recommended by Boeing to make use of it. It is then a question of HOW you do that. It is that question that we are unravelling here.
Cheers
y
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Old 2nd Jun 2019, 21:42
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If you have nothing else then referring to GPS speed is an excellent idea
Absolutely - if youíve got nothing else, then you use whatever you can. But Airbus gives me a procedure to follow, and a set of pitch/thrust tables which will establish the jet within a couple of knots of the speed itís supposed to be at. Sure, we can back it up with a check of the GS, but thatís going to be secondary.

don't forget you are hand flying so its not easy to judge pitch to within 1 degree
Sorry, I disagree. Any competent pilot should have no trouble setting and maintaining an attitude to within 1 degree. Otherwise just maintaining level flight will be a problem, and a raw data ILS will be a bit of a dogís breakfast.
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Old 2nd Jun 2019, 22:02
  #90 (permalink)  

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Originally Posted by Capn Bloggs View Post
Whatever.

I'll use the GPS down final, you use your tables.
I will do what ever my company SOPs tell me to do. I sure as hell am not going to make up my own checklist.
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Old 2nd Jun 2019, 22:16
  #91 (permalink)  
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Originally Posted by L337 View Post
I will do what ever my company SOPs tell me to do. I sure as hell am not going to make up my own checklist.
hi there L337
unless you fly a non Airbus or Non Boeing your SOPS tell you to refer to GS
Boeing STEP 4 737 QRH UAS
4 Crosscheck the IRS and FMCgroundspeed and winds to determine airspeed accuracy if indicated airspeed is questionable.
Cheers
y
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Old 2nd Jun 2019, 22:36
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Originally Posted by yanrair View Post

unless you fly a non Airbus or Non Boeing your SOPS tell you to refer to GS
Sorry, this is not true. The A330/A340 FCOM has procedures for Unreliable Speed Indication, IAS Disagree, and All ADR Off - and NOWHERE does it make any mention of groundspeed. Nor does the FCTM.
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Old 3rd Jun 2019, 09:05
  #93 (permalink)  
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Originally Posted by itsnotthatbloodyhard View Post


Sorry, this is not true. The A330/A340 FCOM has procedures for Unreliable Speed Indication, IAS Disagree, and All ADR Off - and NOWHERE does it make any mention of groundspeed. Nor does the FCTM.
Ah! OK, Body. Point taken if so. Another posting here from an Airbus guy spoke of "reviewing IRS GS data" as part of the procedure. I will have to try and find it. Any other Airbus people out there confirm of deny?
All I can say is that next time we have an AF 447, and we will, lack of that excellent GS data in your toolbox will be a terrible shame!.
It happened to me yesterday on my boat actually. Sailing along in fog with no visibility at 6knots indicated boat speed. . Boatspeed went to zero instantly - weed I guess = bird strike on plane? Blocked pitot due ice? No way of verfirying by visual reference due to fog.
But, GPS reading 6 knots still so I knew we hadn't really stopped and knew which of the two was telling the truth.
And the engine was still at 2000 revs and, there was no current at the time, aka wind to us pilots!
Now if I had not looked at GS I would. have had a reasonable idea that we were still doing 6 knots since the steady state conditions still existed, and the engine was still going at 2000 revs. But the GS confirmed it and was quite nice to have.
The rest of the trip back was done on GS only. And because I knew the tidal steams - up to 3 knots (half my boat speed at times so equivalent to 300 its wind in a jet) I knew my boat speed too. All the time very accurately.
But I digress as usual because I learned to fly when Pontius was a pilot and pilots were real men. No ladies then. Back to my therapist now.
ps I did fly 737-800 sim. until a year or so ago as a TRE examiner so not utterly senile or out of date.
And I can still an inverted circuit in a 737 sim - taught to me by a Red Bull ace of my acquaintance !
Happy flying
Y
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Old 3rd Jun 2019, 09:10
  #94 (permalink)  
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Originally Posted by L337 View Post
I will do what ever my company SOPs tell me to do. I sure as hell am not going to make up my own checklist.
Good thing for Sully he did not follow company procedures and for all the passengers aboard. Or the BA 747 with quadruple engine failure which wasn't in any book. Had to improvise the whole thing. And.............loads of others. Sioux City comes to mind.
Yes of course you use company procedures but as an experienced airline pilot you bring much more to the party than reading checklists that may or may not reflect your actual situation. I think it is called airmanship or some other archaic word like that!
Cheers and thanks for the post.
Y
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Old 3rd Jun 2019, 10:37
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Except you don’t need to improvise in the case of an unreliable airspeed procedure. Just maintain pitch and thrust it works just fine. Then and only then a quick check at your GPS speed....Yeah if you want. Not required by Airbus QRH tho.
When I fly raw data departure, I always set 10 degrees pitch and CLB thrust and the speed always sit at 250kt +-5kt. It’s super accurate and very easy to remember the pitch/ thrust setting. At least approximately enough to fly 1 or 2 min safely to give time to the PM to find the accurate pitch and thrust setting without overspeeding or stalling.
Most of the time, If you do nothing the plane will keep flying perfectly fine. Relax and take your time.

Last edited by pineteam; 3rd Jun 2019 at 10:50.
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Old 3rd Jun 2019, 11:26
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Originally Posted by yanrair View Post
Good thing for Sully he did not follow company procedures and for all the passengers aboard. Or the BA 747 with quadruple engine failure which wasn't in any book. Had to improvise the whole thing. And.............loads of others. Sioux City comes to mind.
Yes of course you use company procedures but as an experienced airline pilot you bring much more to the party than reading checklists that may or may not reflect your actual situation. I think it is called airmanship or some other archaic word like that!
Cheers and thanks for the post.
Y
yanrair

You are taking reference to "catastrophic" events and compare them to a simple blocked pitot or other system failure creating a single airspeed indicator to fail(or 2)?

You are NOT on the path to good airmanship, you are just heading in the right direction to deliver your license to the nice man behind the counter. IF there are no published procedures for a certain failure(e.g. the ones you mentioned), OF course you revert to experience and good airmanship.
If the CAPTAIN deems it necessary to deviate from procedures it is his right to do so. BUT afterwards he will need to explain what(and why) he did to the nice guys on the green table.
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Old 3rd Jun 2019, 13:50
  #97 (permalink)  

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For some, knowing the book and how to use it is just un-airman like. Swallow it, children of iBooks.
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Old 3rd Jun 2019, 20:01
  #98 (permalink)  
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Originally Posted by pineteam View Post
Except you donít need to improvise in the case of an unreliable airspeed procedure. Just maintain pitch and thrust it works just fine. Then and only then a quick check at your GPS speed....Yeah if you want. Not required by Airbus QRH tho.
When I fly raw data departure, I always set 10 degrees pitch and CLB thrust and the speed always sit at 250kt +-5kt. Itís super accurate and very easy to remember the pitch/ thrust setting. At least approximately enough to fly 1 or 2 min safely to give time to the PM to find the accurate pitch and thrust setting without overspeeding or stalling.
Most of the time, If you do nothing the plane will keep flying perfectly fine. Relax and take your time.
Dear Pineteam
Thats what I have been saying all along, with the addition of taking GS into account too. At the time of failure maintain status quo and if it was working a moment ago, it still will. And will continue to do so for a time. AF 447 for example.
GS extends your 1-2 minutes almost infinitely . Even if you don't know the wind ( which you will) In your scenario settle at 250 its GS with pitch and power accordingly. Unless you have a headwind or tailwind in excess of 50 kts you are perfectly safe. Your IAS must be in the range 200-300 which are both safe. Neither too fast nor too slow. And if the wind is stronger than that you will surely know about it and take it into account.

In AF 447 if GS had been kept at 450 kts and level flight , nothing could have gone wrong. nothing. No need for immediate reference to tables from manufacturer which need to be accessed and understood.

The major accidents due to UAS all get into major speed errors way outside these sort of numbers due to total disorientation and confusion. Yet right there is your GS to help stabilise things.
Y

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Old 3rd Jun 2019, 20:14
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Originally Posted by tomuchwork View Post
yanrair

You are taking reference to "catastrophic" events and compare them to a simple blocked pitot or other system failure creating a single airspeed indicator to fail(or 2)?

You are NOT on the path to good airmanship, you are just heading in the right direction to deliver your license to the nice man behind the counter. IF there are no published procedures for a certain failure(e.g. the ones you mentioned), OF course you revert to experience and good airmanship.
If the CAPTAIN deems it necessary to deviate from procedures it is his right to do so. BUT afterwards he will need to explain what(and why) he did to the nice guys on the green table.
Dear Tomuchwork

I am indeed talking catastrophe since we are discussing how to avoid AF 447 and ET and Lionair and lots of others where the crew did or may have lost complete control due to overload, confusion and stress. It was a simple blocked pitot system that caused AF 447 which the pilots managed to turn into a catastrophe because - they didn't fully appreciate how to handle loss of airspeed information coupled with multiple warnings and AP disconnect. In ET and Lionair it looks like a failure of AOA started a chain of events that led to major overspeed of the plane. Not talking MCAS here, just loss of airspeed info and stick shakers.
In AF 447, having pitched the aircraft to some 15 deg. nose up and losing almost all airspeed, there was only one way out, and this is not covered in any manual I can assure you. They had about 15 seconds to realise, put plane into a very steep dive of about 40 degrees to break the stall and gently pull out over perhaps 30 seconds so as not to pull the wings off. Something like that, and it was not going to happen since first of all they had no idea what was going on, but IF THEY HAD it was all over anyway since to break a deep stall you have to dive, in the dark over the ocean decisively.In short you need to understand how to get out of a deep stall.
I confess that we may be into thread creep here so apologies. My point is, that "simple blocked pitot or other system failure creating a single airspeed indicator to fail(or 2)?" has caused many a crash.
Y
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Old 3rd Jun 2019, 20:26
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QUOTE
If the CAPTAIN deems it necessary to deviate from procedures it is his right to do so. BUT afterwards he will need to explain what(and why) he did to the nice guys on the green table.

REPLY
You seem to have a very jaundiced view of airline management - perhaps from bitter personal experience?
In my last job, I was the man who sat behind the green table, actually red. And if any of our captains had made a decision which he carried out in the belief it was the right thing to do (not negligent or foolhardy) but was wrong with the benefit of hindsight, then it was treated as such and no action would be taken except to learn from it and publish it to the rest of the pilots as a learning exercise. it is in this manner that we all learn. One of the mysteries for me is how following Lion Air, that every pilot on the MAX worldwide wasn't an expert in all the factors that led to the loss of that plane. Publishing the Boeing Bulletin is not enough.

If a captain deliberately behaved in a manner that was against training, airmanship, common sense and was done with malice aforethought, that would be a different story. An example might be disregarding a hard GPWS warning.
Y
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