Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Using GPS ground speed to resolve Unreliable Airspeed

Tech Log The very best in practical technical discussion on the web

Using GPS ground speed to resolve Unreliable Airspeed

Old 24th May 2019, 22:36
  #1 (permalink)  
Thread Starter
 
Join Date: Nov 2007
Location: dublin
Posts: 1
Using GPS ground speed to resolve Unreliable Airspeed

There have been a few comments about the usefulness or otherwise of GPS ground speed to safely fly when Unreliable airspeed, speed disagree and stick shakers rattling , at the same time as crazy high (or low) IAS indications are present. I.e. to resolve apparent chaos. AF447 and ET /LIONAIR are but three examples.

i contend that not only is G/S useful- it is the solution in most cases.

Let the conversation begin!
For start, I hope nobody disagrees that you can fly a perfect circuit using GPS only with ALL pressure instruments including IAS and altimeters not working. ?
Happy flying
Y
yanrair is offline  
Old 24th May 2019, 22:58
  #2 (permalink)  
 
Join Date: Apr 2010
Location: IRS NAV ONLY
Posts: 831
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.
FlyingStone is offline  
Old 25th May 2019, 00:05
  #3 (permalink)  
 
Join Date: Oct 2012
Location: Australia
Posts: 78
Originally Posted by yanrair View Post
For start, I hope nobody disagrees that you can fly a perfect circuit using GPS only with ALL pressure instruments including IAS and altimeters not working. ?
I'm not sure how you define "perfect", but for me it would include following the manufacturer's recommendations on airspeed - especially during takeoff and landing. Say you're in a C172 or PA28 with a 20kt headwind straight down the runway. If you just assume that airspeed = ground speed, then you're going to be taking-off 20kts faster (airspeed) than recommended - except that you won't be able to because the plane will insist on flying before that point. Then you turn downwind, and instead of the normal 80kts-ish downwind you're doing more like 60kts - which is not a stall, but is not much fun either. More challenging is landing; if you come in at 20kts above recommended approach speed, it's going to happily fly straight off the other end of the runway. That is, if you haven't already torn the flaps off and crashed by lowering full flap at 20kts above the maximum flap speed...

Of course, if you know your power settings then you can fly a perfectly acceptable circuit without any air or ground speed indication, and in either of those planes you can feel your airspeed just in the control response. Or if you have accurate wind readings and a bit of time to do the maths then you can convert ground speed into airspeed. However, assuming that the pilot knows the power settings, can feel the response, or has accurate wind readings and spare time may be a mistake.


Slatye is offline  
Old 25th May 2019, 00:18
  #4 (permalink)  
 
Join Date: Apr 2004
Location: Planet Earth
Posts: 1,816
Display of AOA is the best possible back up
stilton is offline  
Old 25th May 2019, 00:49
  #5 (permalink)  
Moderator
 
Join Date: Aug 2006
Location: Ontario, Canada
Age: 58
Posts: 4,204
Well, I've had my airplane flying 11 MPH backward, according the GPS groundspeed (It was a windy day), so with that in mind, I'll continue to use the airpseed indicator for airspeed information.

I do remember test flying a Tiger Moth following maintenance. It had three airspeed indicators, one in each cockpit, and a vane device on the wing strut. They all differed by about 10 MPH to each other, so I ignored them all, and just flew by feel, it was fine. In hindsight, I think the vane on the wing seemed the most accurate.

I really like the GPS for pointing me home, and telling me when I should expect to be there.
Pilot DAR is offline  
Old 25th May 2019, 00:59
  #6 (permalink)  
 
Join Date: Mar 2002
Location: Seat 0A
Posts: 7,769
Originally Posted by Flying Stone
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.
Hang on. Nobody is suggesting you suddenly find yourself plopped into a scenario with now idea how you got there or what the current conditions were and now had use the GPS.

You will obviously be happily flying along, knowing you are doing the right speed, then the pressure instruments go awry. The GPS GS is a great indicator in that case to keep you flying until you descend or otherwise sort out the problem.

The other scenario is after takeoff. You are starting from a relatively accurate known point: get it cleaned up and maintain 250 GPS GS, allow for the low level wind if you like. Safe as houses.

GPS GS is a fantastic aid if you have no IAS. You could even use your phone GPS speed.

Display of AOA is the best possible back up
In my aeroplane, the FPA is not reliable during a UAS event. I assume that AOA (unless it was just a raw readout from the vane) would also be dodgy.
Capn Bloggs is offline  
Old 25th May 2019, 05:47
  #7 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 12,227
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.
ShyTorque is offline  
Old 25th May 2019, 05:57
  #8 (permalink)  
 
Join Date: Mar 2002
Location: Seat 0A
Posts: 7,769
What's wrong with the tried and tested method of: "The correct attitude + correct power setting = correct speed?"
In your 737 at 40t or your 737 at 60t, if you set your UAS parameters at slow speed or high speed the speed outcome will be completely different.
Capn Bloggs is offline  
Old 25th May 2019, 06:21
  #9 (permalink)  
 
Join Date: Apr 2010
Location: IRS NAV ONLY
Posts: 831
Originally Posted by Capn Bloggs View Post
In your 737 at 40t or your 737 at 60t, if you set your UAS parameters at slow speed or high speed the speed outcome will be completely different.
And yet, it's safe, which is what matters in the end, not how accurately you can fly the speed you can't even see or whether you will climb or descent.

This is Boeing's take on this:

The memorized pitch and thrust setting for the current configuration (flaps
extended/flaps up) should be applied immediately with the following
considerations:
• The flaps extended pitch and thrust settings will result in a climb.
• The flaps up pitch and thrust settings will result in a slight climb at light
weights and low altitudes, and a slight descent at heavy weights and
high altitudes.
• At light weight and low altitude, the true airspeed will be higher than
normal, but within the flight envelope. At heavy weight and high
altitude, the same settings will result in airspeed lower than normal
cruise but within the flight envelope.
• The goal of these pitch and thrust settings is to maintain the airplane
safely within the flight envelope, not to maintain a specific climb or
level flight.
• The current flap position should be maintained until the memory pitch
and thrust settings have been set and the airplane stabilized. If further
flap extension/flap retraction is required refer to PI-QRH Airspeed
Unreliable table.
Originally Posted by Capn Bloggs View Post
In my aeroplane, the FPA is not reliable during a UAS event. I assume that AOA (unless it was just a raw readout from the vane) would also be dodgy.
If installed, AOA readout normally is directly from the vane.

Last edited by FlyingStone; 25th May 2019 at 06:28. Reason: Additional content
FlyingStone is offline  
Old 25th May 2019, 06:29
  #10 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 12,227
Originally Posted by Capn Bloggs View Post
In your 737 at 40t or your 737 at 60t, if you set your UAS parameters at slow speed or high speed the speed outcome will be completely different.
But aren't pilots paid to know the difference?
ShyTorque is offline  
Old 25th May 2019, 06:41
  #11 (permalink)  
 
Join Date: Mar 2002
Location: Seat 0A
Posts: 7,769
Originally Posted by Small Cog
Let me guess, child of the magenta line?
Let me guess, a luddite that has never actually tried a full-blown "I have no idea what my speed is" approach?

You lot should try to fly an approach in a jet based solely using the speed/power/attitude tables. Then fly it using the GPS speed and the wind from the tower. Chalk and Cheese.

Originally Posted by FlyingStone
And yet, it's safe, which is what matters in the end, not how accurately you can fly the speed you can't even see or whether you will climb or descent.
Not IMO. Those numbers and procedures are obviously designed for the heat of the moment to prevent you from stalling. A light jet at low level will still accelerate quickly at the recommended UAS parameters and could well end up like the Ethiopians. What's your power setting for 250KIAS at low level?

Originally Posted by Shytorque
But aren't pilots paid to know the difference?
Are you a 737 pilot? Do you know the difference?
Capn Bloggs is offline  
Old 25th May 2019, 07:38
  #12 (permalink)  
 
Join Date: Dec 2001
Location: Brisvegas
Posts: 2,724
https://support.garmin.com/en-AU/?fa...Uv1QyoxITW2vZ6

Flying a circuit on GPS altitude bears no resemblance to "altitude".
Icarus2001 is offline  
Old 25th May 2019, 07:41
  #13 (permalink)  
 
Join Date: Mar 2002
Location: Seat 0A
Posts: 7,769
Originally Posted by Cog
What next? Engine fire ... go look for a big rain cloud just in case the fire extinguishers don’t work when they are initiated?
Whatever.

I'll use the GPS down final, you use your tables.
Capn Bloggs is offline  
Old 25th May 2019, 08:03
  #14 (permalink)  
 
Join Date: Feb 2001
Location: The Winchester
Posts: 5,364
My two pence worth if, as I understand it, there's an argument here about setting the gross pitch power Boeing figures vs. "simply" setting well remembered pitch power immediately you recognise an unknown airspeed situation..

As I read it (from our FCTM and other documentation) Boeing's logic behind not wanting ace pilots setting their committed to memory pitch and power figures for, e.g. for 250 knots 'cos they think they are at 250 knots and S&L is because by the time they recognise the situation they might not be at 250 knots and they might not be in level flight....so that's why they came up with the "it's safe" figures

Of course after the trouble shooting is done we usually arrive by way of a checklist to the weight/pitch/ power etc tables and then fly the machine that way.... so eventually you get to use both techniques (and so honour is satisfied, magenta line, old school or both).

Last edited by wiggy; 25th May 2019 at 08:14.
wiggy is online now  
Old 25th May 2019, 08:28
  #15 (permalink)  
 
Join Date: Aug 2013
Location: Not lost, but slightly uncertain of position.
Posts: 208
Originally Posted by Capn Bloggs View Post
Hang on. Nobody is suggesting you suddenly find yourself plopped into a scenario with now idea how you got there or what the current conditions were and now had use the GPS.
Well, I got the impression that the crew of AF447 did get plopped into their scenario without any idea of how they got there. Trucking along for 10 hours straight, do you constantly keep track of your GND speed? It can change with values that exceed your A/C limits or stall speed, over a very short period of time i you are flying close to the coffin corner.

In my aeroplane, the FPA is not reliable during a UAS event.
Thats because the relation between FPA (INU/INS), Pitch and AOA is only perfectly constant and reliable in a no wind environment, flying in a straight line (1g). However, the FPA is a very reliable means of showing level flight regardless of the status of the AS indicators.

I assume that AOA (unless it was just a raw readout from the vane) would also be dodgy.
AOA is raw readout in either units or degrees. It is only dodgy if the vane is broken like on the MAX. AOA is most useful for aircraft where the total mass can vary a lot during flight, since it will always show you how far you are from stalling. Aircraft with AOA indicators normally have a fixed AOA value to be flown during approach regardless of their weight, thereby changing the status of the AS indicator to a B/U instrument only (during approach).

GPS is nice to point you home, give military pilots great weapon precision and such, but my suggestion is always to be able to fly your aircraft via the control and performance concept, as its the only proven concept that will get you safely on the ground. But there is a catch. It requires propper skills and sufficient training, a thing that magenta pilots probably don't have/get.
F-16GUY is offline  
Old 25th May 2019, 09:03
  #16 (permalink)  
 
Join Date: Sep 2011
Location: Belgium
Age: 60
Posts: 136
While AOA is one of the best alternatives, remember that the latest crashes are due to failing AOA sensors triggering the events that end up flying the aircraft into the ground.

The very best alternative is to equip all aircraft with a crazy stupid alternative that is completely disconnected from the aircraft like a Dynon D3 pocket panel and revert back to basic flying skills..
Vilters is offline  
Old 25th May 2019, 10:11
  #17 (permalink)  
 
Join Date: Apr 2010
Location: IRS NAV ONLY
Posts: 831
Originally Posted by Capn Bloggs View Post
Not IMO. Those numbers and procedures are obviously designed for the heat of the moment to prevent you from stalling. A light jet at low level will still accelerate quickly at the recommended UAS parameters and could well end up like the Ethiopians. What's your power setting for 250KIAS at low level?
A 737 will go to Vmo at 10deg pitch and 80% N1 with flaps extended? Or 4 deg pitch and 75% N1 in clean configuration? Even at OEW and a bit of fuel to run the engines, I very much doubt it.

Why do you want to fly exactly at 250 KIAS? It is safe, for sure, but so are UAS pitch/power settings.

I would be very cautios when operating the aircraft outside of manufacturer's recommendations and procedures, especially when it directly contradicts them. A lot of work in the aviation industry went into developing robust UAS procedures post AF447, and they are much better now that they used to be. And despite all the media propaganda, I still believe Boeing engineers and test pilots have more (abnormal) aircraft/sim time and know more about UAS than many of us together.

Boeing FCTM says (my bold):
Memory items for target pitch and thrust must be accomplished as soon as it is suspected that airspeed indications are incorrect.
Good luck explaining to a lawyer why your pitch/thrust settings were better than manufacturer approved procedure, if things end up sideways.
FlyingStone is offline  
Old 25th May 2019, 10:23
  #18 (permalink)  
 
Join Date: Nov 2005
Location: Zulu Time Zone
Posts: 650
As I read it (from our FCTM and other documentation) Boeing's logic behind not wanting ace pilots setting their committed to memory pitch and power figures for, e.g. for 250 knots 'cos they think they are at 250 knots and S&L is because by the time they recognise the situation they might not be at 250 knots and they might not be in level flight....so that's why they came up with the "it's safe" figures
Wiggy is correct plus they also state "at any and all weight/altitude combinations, the aircraft will accelerate from low speeds, and slow from a high speeds.... At light weight and low altitude, the true airspeed will be higher than normal, but within the flight envelope ".
oggers is offline  
Old 25th May 2019, 10:25
  #19 (permalink)  
 
Join Date: Jan 2019
Location: Surrey
Posts: 248
Call me old fashioned (I could easily be defined as a child of the magenta, in fact) but there’s a reason why the manufacturer put in the QRH a procedure for UAS and make it a memory item.

Setting a sensible pitch / power setting, and then following the QRH procedure for trouble shooting has to be the safest option.
Busdriver01 is offline  
Old 25th May 2019, 10:51
  #20 (permalink)  
 
Join Date: Mar 2002
Location: Seat 0A
Posts: 7,769
Wiggy, I agree.

Originally Posted by F16 guy
Trucking along for 10 hours straight, do you constantly keep track of your GND speed?
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!

At light weight and low altitude, the true airspeed will be higher than normal, but within the flight envelope
Precisely, and where the Ethiopians found themselves immediately prior to the final dive: at Vmo. I wonder what the GPS GS was then...

Originally Posted by F16
In my aeroplane, the FPA is not reliable during a UAS event.
Thats because the relation between FPA (INU/INS), Pitch and AOA is only perfectly constant and reliable in a no wind environment, flying in a straight line (1g). However, the FPA is a very reliable means of showing level flight regardless of the status of the AS indicators.
Just telling you what my FCOM says.

Originally Posted by Flying Stone
Why do you want to fly exactly at 250 KIAS? It is safe, for sure, but so are UAS pitch/power settings.
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...

Originally Posted by Busdriver
Setting a sensible pitch / power setting, and then following the QRH procedure for trouble shooting has to be the safest option.
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.
Capn Bloggs is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.