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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 26th May 2019, 05:54
  #41 (permalink)  
 
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AF447 should not be brought in because the crew was overwhelmed. They didn't see anything not even the pitch that resulted fro their actions forget the GPS GS. UAS procedure is based on a philosophy to manage the aircraft speed within a safe band, accuracy is not the purpose nor is it possible. It is similar in Boeing or Airbus. After having applied recommended procedure GPS GS may be referred for awareness but it won't be wise to fine tune anything. Airbus has come out with back up speed(different from B/U SS) obtained by applying lift equation to GW, AOA, CG. It's there in A350 and optionally available on other models. If fitted then it's as simple as AP/FD TCAS. You do nothing. Just Say thank you.
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Old 26th May 2019, 10:20
  #42 (permalink)  
 
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Originally Posted by itsnotthatbloodyhard View Post


Not in any A330 I’ve ever flown, unless your ‘normal cruise’ is down around F200. For F350, M0.8, ISA+10, 205t (as per AF447), the tables I’m looking at indicate about 95.5% N1. That’s for the CF6 - would the RB211 be that much different?
ACMS quotes accurate numbers above for the RR Trent 700 powered A333 in cruise around F370.
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Old 26th May 2019, 10:59
  #43 (permalink)  
 
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It seems as if we are assuming we don't know the actual windspeed, when this can be found by asking the nearest ATC unit for their ground windspeed, then adding the correction of twice that and add 30 degrees to find the wind velocity above 1000 ft.
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Old 26th May 2019, 11:17
  #44 (permalink)  
 
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Just have an idea of the pitch and power you need, it’s not rocket science. Plus on Airbus the thrust would be locked so you don’t even need to worry about it unless you are in a climb or descent.
A320: Above FL 300: 80% N1 pitch 2 degrees up
between Fl200 and Fl300 : 70% N1 & same pitch
Below Fl200 60%: N1 pitch 3 degrees up then 4 up below Fl100.
Just some rough value from what I saw during flights. With these I might climb or descent gently but I won’t stall nor overspeed and it gives me time to reach the QRH to do fine tunning.


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Old 26th May 2019, 11:22
  #45 (permalink)  

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scifi, that's pushing too far, it is not a hot air baloon.

The best solution is to at first do nothing and check all the remaining parameters, to be used later as a reference and self-check over and over again.

This thread inspires me to include GS in that scan the next time, and it might had prevented a ckup I managed to create in the last session. Having said that, anything additional must be included carefully not to compromise the original underlying skill.

Last edited by FlightDetent; 26th May 2019 at 13:41.
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Old 26th May 2019, 12:44
  #46 (permalink)  
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The best solution is to at first do nothing and check all the remaining parameters
I like that answer! I observe too often that in cruise flight something will appear to "happen" or change, and a pilot thinks that they need to apply their cat like reflexes to compensate for it. Sometimes it's better just to have already been aware, and then continue to be aware, perhaps with an added element to consider. As time passes, and other factors require a change, then maybe compensation, or a changed plan is going to be needed. Maybe, nothing (other than a written up snag) will be required.

Which reminds me of one of my learning events. I was first time left seat ferrying a Twin Otter with a very experiences mentor pilot friend. We were leaving Cairo southbound, and low altitude nav aids were few and far between. This particular Twin Otter did not have a DG at all, it had two slaved RMI's and the magnetic compass (which is not remarkably useful for flying a heading in a Twin Otter). We had noticed on previous legs that the RMI's would drop a flag an quit, seemingly randomly, so becoming useless for flying a heading. We agreed that whomever had a working RMI would fly (no auto pilot, hand flying the whole trip anyway). So, when a half hour into a 9 hour leg, my RMI dropped a flag, I wasn't really eager to surrender flying just yet, I was enjoying myself! My mentor friend was consumed with a marine "Sat Nav" device he had brought on the trip. This was before the days of GPS - the time before magenta lines, so it was charts and heading indicators, other than Bill's occasional fixes on his Sat Nav. He was happy watching the Sat Nav, and probably had little interest in flying anyway, so I kept flying... but how was I going to hold a heading? I could look across at his RMI, but that was not a really good long term solution. As it was a very clear day, I could fly by ground reference for a while, but in that part of the world, sand is sand, so there were not many features to pick on the horizon.

So I flew on, taking my time, and thinking. During this period, I found a solution. I considered it, verified it with occasional glances to Bill's RMI, and applied it. I flew a perfect track. Eventually, Bill noticed that my RMI had dropped a flag, he commented without alarm. He asked if I was okay continuing flying, and I said I was. Over the next number of hours, he used the Sat Nav to confirm that my track was right on, and eventually asked me how I was doing it. I told him I'd tell him later.

25 flying hours later, (all but the final leg of which I was offered left seat), and many more RMI failures, I had kept my technique to myself, and he seemed content to ride along, confirming my track, and perfecting his use of the Sat nav for sailing (which interested him more than flying a Twin Otter). We arrived in Maseru and delivered the plane to Air Lesotho. At dinner he finally said: "Okay, you gotta tell me how you were doing that, you were flying perfect tracks for hours with no practical heading indicator." I explained that while flying with the RMI card stopped, I noticed that the RMI slaving meter would show the failing attempts of the remote compass to slave the card. As long as the RMI failed on the heading I wanted to fly, and I kept the slaving meter centered, to plane followed the heading. He quietly smiled. I learned to determine if something was a problem, before doing something to solve it.
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Old 26th May 2019, 23:29
  #47 (permalink)  
 
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Originally Posted by scifi View Post
It seems as if we are assuming we don't know the actual windspeed, when this can be found by asking the nearest ATC unit for their ground windspeed, then adding the correction of twice that and add 30 degrees to find the wind velocity above 1000 ft.
.
Yeah, I'm sure they would have had an accurate ground wind reading for AF 447.


Never mind that I've seen wind speed over 150kts without hurricane warnings on the ground.


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Old 27th May 2019, 07:30
  #48 (permalink)  
 
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Yes I should have said A330-300 RR powered is 2.5 degrees and 78% N1
I don’t know anything about the P and W or GE 330’s

Smythe—-I’m quoting Pitch attitude on the ADI......not AOA or anything else.
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Old 27th May 2019, 09:21
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Originally Posted by Smythe View Post
Just curious..when you state 2.5 degrees, Is this the AoA of the fuselage or the wings?

On the A333. what is the AoA delta between the fuselage and the wings?
Wing root incidence on the A330 is, I believe, +4.5°.
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Old 27th May 2019, 10:17
  #50 (permalink)  
 
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Originally Posted by Rad Alt Alive
Using Airbus FBW procedures as an example,...
Originally Posted by Rad Alt Alive
There is no mention anywhere, in any of our manuals, about accessing the GPS data to determine GS. There is no mention about asking ATC what our GS is. No mention of GS, full stop!
Originally Posted by Rad Alt Alive
I would instead encourage that they just follow the specific manufacturer’s procedures.
Interestingly, today I was reading the report into an A320 Pitot Blockage event, which included a stall warning on final approach. Page 64 and on (a copy of the Airbus FCOM Abnormal procedure for UAS) references using GPS altitude and speed:

"GPS Altitude...Display on MCDU"
"Refer to GPS Altitude..." to the extent that it can be used to maintain level flight.
"Crosscheck all speed indications..."
"Alternate sources may be used to evaluate the data (from the reliable ADR):
-GPS altitude
-GPS and IRS ground speeds, taking into account altitude and wind effects".

Full stop indeed.



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Old 27th May 2019, 14:43
  #51 (permalink)  
 
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The airspeed unreliable memory items and checklist were developed by Boeing from recommendations / guidelines published by Flight Safety Foundation.
These guidelines included,
Procedures should provide information on altitude and power settings that enable crew to maintain the aircrafts flight parameters within normal limits during flight with unreliable airspeed events for all phases of flight.
Procedures should address the availability and use of independent alternate sources of airspeed information (e.g. GPS, inertial, angle of attack etc.)
Procedures should include memory items for critical immediate action steps

The recommendations also provided an Airspeed Unreliable Generic Template,
1. Disconnect Automation - Rationale: automation may be reacting to airspeed indications that may not be correct, so it must be disconnected
2. Stabilse the aircraft with the provided pitch attitude and thrust - Rationale: setting memorised pitch attitude and thrust settings stabilise the aircraft in climb or cruise as applicable. if in a descent, the aircraft should be levelled off and the cruise setting used.

There in ends the recommended memory items, reference items of the checklist then direct the amongst other items to set an applicable attitude and thrust. Boeing aircraft this information is found in the QRH PI section.

In summary
The initial pitch and power settings of the memory items on a Boeing aircraft are designed to keep the aircraft "safe" until such time as the accurate Pitch and Power setting s can be extracted from the QRH
PI section.

Regarding GPS ground speed, the elephant in the room is GPS jamming, it is becoming more and more prevalent in particular regions of the world so much so that IATA has recently issued an operator alert on the subject.
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Old 27th May 2019, 15:17
  #52 (permalink)  
 
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Capn Bloggs
"GPS Altitude...Display on MCDU"
"Refer to GPS Altitude..." to the extent that it can be used to maintain level flight.
"Crosscheck all speed indications..."
"Alternate sources may be used to evaluate the data (from the reliable ADR):
-GPS altitude
-GPS and IRS ground speeds, taking into account altitude and wind effects".

Full stop indeed.
Why did you not quote the full Airbus Procedure as what you quoted would be after the stabilization of the aircraft. You can twist anything to make a point!
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Old 28th May 2019, 10:46
  #53 (permalink)  
 
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Originally Posted by sleeve of wizard View Post
Regarding GPS ground speed, the elephant in the room is GPS jamming, it is becoming more and more prevalent in particular regions of the world..
Good point, yes it is.
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Old 28th May 2019, 17:47
  #54 (permalink)  
 
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Can GPS measure dynamic pressure ?

Did you say no ?

There's your answer then.
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Old 28th May 2019, 21:30
  #55 (permalink)  
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Can GPS measure dynamic pressure ?
Hold the touchscreen out the window into the airflow? Hmmm, a new app for phones, the touch screen is the pitot tube!
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Old 29th May 2019, 03:57
  #56 (permalink)  
 
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Guys if it was a good idea to use GPS or IRS ground speed for unreliable airspeed it would already have been implemented in the unreliable airspeed procedure.
In my home base, it's common to have on final a tail wind of more than 20 kt around 2500 feet AGL and a headwind on landing... The wind speed and direction change dramatically.
GPS is a great tool, I used it as my primary nav instrument back in the days flying in the bush; But I would definitely not use it as a primary tool to recover from an unreliable airspeed.
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Old 29th May 2019, 22:40
  #57 (permalink)  
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Originally Posted by Capn Bloggs View Post
Wiggy, I agree.


No, what's the point? When you go UAS, then note your GPS GS. Maintain it until you work out what's going on. If you pull up to 4° and pull the power back to 75% (Flying Stone/737 UAS?) at FL370, you'd better watch that GPS GS closely because it'll be reducing!


Precisely, and where the Ethiopians found themselves immediately prior to the final dive: at Vmo. I wonder what the GPS GS was then...


Just telling you what my FCOM says.


I didn't say anything about "exactly" 250KIAS. 230-270 GPS GS who cares? At least it's not Vmo with full nose down MACS trim while maintaining the UAS "it's safe" memory numbers while your PM comes up with some numbers from the back of the QRH...


I agree. My point is that GPS GS is an invaluable aid while doing that, especially down low on approach or after TO when wind effect is low/known.
dear Captain Bloggs
i started this thread to see what proportion of readers realized that GS is not just useful, but makes UAS.an easy exercise.
You seem to be one of “ the few” believers!
Y
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Old 29th May 2019, 22:47
  #58 (permalink)  
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Originally Posted by FlyingStone View Post
Personally, I'd stick with the approved procedures for unreliable airspeed.

Your GS is 500kts. Are you safe? Could be TAS of either 650 or 350 with a 150kt wind. Neither of which will keep you flying for long at cruising FL.
but since you know your wind speed to within 5 knots this doesn’t happen. Your flight plan is accurate +/- 5 kts and even synoptic chart within say 15. And anyway you know the GS at time of failure and it ain’t going to change in the next ten minutes which is how long it too AF447;to lose all sense of speed ending up at less than 110 kts. If they had set pitch power or even left pitch power at existing settings and maintained GS 450 kts ,:it would have been time to ding for a cup of tea while considering further actions. Ok perhaps no tea.
Y
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Old 29th May 2019, 23:03
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Originally Posted by pineteam View Post
Guys if it was a good idea to use GPS or IRS ground speed for unreliable airspeed it would already have been implemented in the unreliable airspeed procedure.
In my home base, it's common to have on final a tail wind of more than 20 kt around 2500 feet AGL and a headwind on landing... The wind speed and direction change dramatically.
GPS is a great tool, I used it as my primary nav instrument back in the days flying in the bush; But I would definitely not use it as a primary tool to recover from an unreliable airspeed.
Perfect.
2500 ft tailwind 20
airfiel wind headwind 20
So your landing GS will be 20 kts less than V Ref. Say VRef 100, then fly whole approach at 120 GS. Your IAS will be 140 at 2500 and 100 on landing. Just what you were looking for- no? A perfect touchdown speed. Not approx.
more commonly one has reducing headwind during final descent. Say 30 kts HW at 1000 ,and 5 kts at touchdown. VRef 100.
Fly whole approach at 105 kts. GS. You will land at EXACTLY 100 VRef!
this is precision flying- not guesswork. Clearly you use manufacturer Pitch/power tables too to give you a clue but then refine the outcome using GS.
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Old 29th May 2019, 23:06
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Originally Posted by wiggy View Post
Good point, yes it is.
GPS has jammed very rarely and if it does every plane airborne is going to be in some difficulties.. but for the GPS To fail just as your failed instruments is unthinkable statistically
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