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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 |
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. |
Originally Posted by yanrair
(Post 10478976)
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. |
Display of AOA is the best possible back up |
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. |
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.
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 |
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. |
What's wrong with the tried and tested method of: "The correct attitude + correct power setting = correct speed?" |
Originally Posted by Capn Bloggs
(Post 10479103)
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.
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
(Post 10479035)
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.
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Originally Posted by Capn Bloggs
(Post 10479103)
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.
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Originally Posted by Small Cog
Let me guess, child of the magenta line?
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.
Originally Posted by Shytorque
But aren't pilots paid to know the difference?
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https://support.garmin.com/en-AU/?fa...Uv1QyoxITW2vZ6
Flying a circuit on GPS altitude bears no resemblance to "altitude". |
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?
I'll use the GPS down final, you use your tables. |
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). |
Originally Posted by Capn Bloggs
(Post 10479035)
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.
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. 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. |
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.. |
Originally Posted by Capn Bloggs
(Post 10479118)
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?
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. |
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 |
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. |
Wiggy, I agree.
Originally Posted by F16 guy
Trucking along for 10 hours straight, do you constantly keep track of your GND speed?
At light weight and low altitude, the true airspeed will be higher than normal, but within the flight envelope
Originally Posted by F16
In my aeroplane, the FPA is not reliable during a UAS event.
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.
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.
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Originally Posted by Capn Bloggs
(Post 10479246)
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!
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.
Originally Posted by Capn Bloggs
(Post 10479246)
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...
During takeoff roll, the engines stabilized at about 94% N1, which matched the N1 Reference recorded on the DFDR. From this point for most of the flight, the N1 Reference remained about 94% and the throttles did not move. Next time you're in the sim, ask the instructor for couple of UAS scenarios at various weights/altitudes and you'll see Boeing's pitch/thrust settings tend to give decent results. |
Posters, the discussion is okay (though let's keep working at being courteous), but this forum may not be the best place for it. Would someone like to suggest the best forum for it, and I'll move it there?
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Pilot DAR, Tech Log?
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1. Fly in any direction for five minutes
2. Fly the reverse course for five minutes 3. Divide displacement over ground (from GPS device) by ten minutes 4. Result is wind velocity YYZjim |
Originally Posted by YYZjim
(Post 10479368)
1. Fly in any direction for five minutes
2. Fly the reverse course for five minutes 3. Divide displacement over ground (from GPS device) by ten minutes 4. Result is wind velocity YYZjim |
Originally Posted by F-16GUY
(Post 10479166)
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.
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. 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. |
Originally Posted by Capn Bloggs
(Post 10479118)
Are you a 737 pilot? Do you know the difference? But I certainly do know the difference between IAS and G/S because wind velocity awareness is critical to carrying out my present job safely. I was taught from square one to learn power/attitude settings that give a safe baseline to work from when flying jet powered aircraft and for all other types. For a couple of decades I was responsible for teaching others to do the same. It appears from some accident reports that more recent training might unfortunately not be so comprehensive. :( |
Other than encouraging debate, the assertion that GS (GPS-derived or otherwise) is better than AoA (essentially what the manufacturers are directing the pilots to concentrate on with their ‘pitch/power’ tables) is so flawed as to be laughable. Using Airbus FBW procedures as an example, the onset of unreliable speed during steady-state cruise requires no immediate action from the pilots. Most likely, thrust will be locked as the FMS detects the anomaly, and the AP will disconnect. This is the aircraft ‘telling’ the pilots that it can’t figure out what to do, but that the last thing that it WAS doing was working, so now it’s over to the pilots. Hopefully then, neither pilot will do ANYTHING; the aircraft FBW will keep the aircraft flying at 1G, at a power that was safe. Then it’s either a matter of calmly and methodically checking the QRH for the suggested pitch and power (if cruising at high levels - Airbus’ take being that at high levels, these events are short term in duration), or - if at lower levels - following the procedure to activate the BUSS (Back Up Speed Scale), where the PFD changes it’s presentation to a very easily interpreted AoA scale. This presentation allows very intuitive operation in all regimes, and during configuration for approach and landing. 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! Jet performance is so great, that their climb and descent capabilities (ie resulting in significant change in TAS and wind velocity over relatively short periods of time) as well as their high cruising speeds at high altitudes (with it’s possible significant change in wind velocity over short to medium distances) make any use of GS in UAS events unwise, ineffective and downright dangerous. For those who might think Bloggs’ assertion is a good idea, I would instead encourage that they just follow the specific manufacturer’s procedures. |
GS is a good tool in the box.
Can be used for reference, but in no way should it take precedence over pitch/power. |
Originally Posted by Small cog
(Post 10479112)
Let me guess, child of the magenta line? When you learnt to fly were you taught somewhere around lesson 1 that in each aircraft configuration the airspeed achieved will be as a result of the power + attitude? Was it not demonstrated too? That’s why the manufactures provide “unreliable airspeed” procedures based upon configuration/weight/altitude/attitude and power setting required. So.........,,, 1. Not magenta line guy ! Actually navigated by Astro sextant on passenger jets until 1978! And have flown 737 all variants, 757. 767. l1011. 747-400. 2. Pitch And power were the only way to fly UAS prior to G/S readout. Still the primary source of info. But GPS refines that info and guarantees that you are in the right safe zone 3 captain bloggs above has it right. We are seeking a safe flight and approach which keeps you well away from stall or excess speed. Guaranteed 4. For years say 1985-1995 many types especially. Tristar used GS on every approach in heavy winds to refine IAS. It then fell out if common use but I’ve seen it used up to recently. Over, but not out...,,, its a fascinating thing that there is one readout on the flight deck- actually three- GS that don’t fail and ensure safe flight but very few believe it. One major airline teaches it- maybe others too. Y |
Simple example if AF447 had maintained existing pitch at 3 deg. NU, 70%N1 power (approx) and GS 460 kts (whatever it was at the time of failure) nothing would have happened. Safe flight for the foreseeable future. If GS fell by 30 kts say, then apply thrust back to 460 GS and readjust power to 72% and so on. If anyone thinks this wouldn’t work perfectly please say why. Cheers and thanks for keeping this thread calm. Y |
Originally Posted by yanrair
(Post 10479649)
Simple example if AF447 had maintained existing pitch at 3 deg. NU, 70%N1 power (approx) and GS 460 kts (whatever it was at the time of failure) nothing would have happened. Safe flight for the foreseeable future. If GS fell by 30 kts say, then apply thrust back to 460 GS and readjust power to 72% and so on. If anyone thinks this wouldn’t work perfectly please say why. |
A330-300 normal cruise is 2.5 degrees pitch and 78% N1 GPS Altitude and GS is very handy BUT use carefully the GS.... |
Okay Posters,
As suggested, here in Tech Log now, continue along as you wish, while new rumours and new news populates the R&N forum... Pilot DAR |
So, I have read this debate many times over the years. It always leads to a hot discussion then degrades to an argument. But I have learned a lot from them. I am a firm believer of looking at all the available data (in the cockpit) to try and form a picture. There really should not be a one-size-fits-all approach to this. Saying that, luckily I (as pretty much everybody else here) have never been in this type of situation aside from the standard recurrent Sim sessions where we are thoroughly briefed on what is going to happen and how to get out of it in the Sim world. And I hope I never do. My issues with the GPS speed thing, is two fold. First, pitch/power should always be first. In many of the transport and corporate aircraft I fly, there are no tables to memorize - a good pilot gains a feel of each type. I am not saying that I remember the exact power numbers that equal airspeed for the various aircraft I fly to maintain an altitude, but I guarantee I know what settings would keep me from stalling or pointing at the ground. Rarely do all the engine indication instruments fail, but I have had ADC (and AHRS) issues in a variety of types. In the C172 pattern/circuit example: I would never fly it looking at the GPS, but I can and have flown/taught visual patterns/circuit without any instruments (covered up) or GPS. Pitch/power/feel of the butt. I have taught many a 15 hr student pilot to do so, but have flown with many 2500 hr pilots who cannot fly a visual approach without some sort of VNAV giving them a profile. The second is the problem with the 3rd dimension. Image a situation where the IAS is decreasing (malfunction), so the pilot pushes down. As the aircraft’s path moves further into the vertical, the 2D GPS GS will decrease. Look at the curve of the hypothetical: strait nose down will theoretically give you zero GS (wind aside). So after maybe about 30’ nose down, as the IAS decreases (malfunction), the pilot pushes nose down and the GS keeps decreasing as the 2D flight path has less of the forward component. Of course the pilot should “sense” the noise and other clues to say he/she is speeding up - but that might be what is missing in this black/white discussion. Again, I really hope I will never encounter a situation such as the AF folks. But I also hope that I would have the experience and wherewithal to not put the aircraft in a nose-dive towards the ground. But at a low-level - man - that would be a really hard situation to live through..... |
A330-300 normal cruise is 2.5 degrees pitch and 78% N1 On the A333. what is the AoA delta between the fuselage and the wings? |
? Are you asking about the angle of incidence between the wing chord and fuselage longitudinal axis?
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A330-300 normal cruise is 2.5 degrees pitch and 78% N1 |
Originally Posted by Rad Alt Alive
Jet performance is so great, that their climb and descent capabilities (ie resulting in significant change in TAS and wind velocity over relatively short periods of time) as well as their high cruising speeds at high altitudes (with it’s possible significant change in wind velocity over short to medium distances) make any use of GS in UAS events unwise, ineffective and downright dangerous.
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angels on a pin
Whether you use GPS GS, TAS, FPA, ATT/PWR is essentially purely a technique to retain SA. Heck, you can use Sound power level, (did that for an accident investigation...) Whatever your preference, FCTM guidance, there are numerous sources of data that can be used.
B777's have lost everything at least twice in operations, and have got close to that a number of other occasions. The B744 has gone blank, for transferred faults, and for idiots in charge of the plane. A B767 and a B757 (condor/Aeroperu) dumped themselves in an untidy heap, while all manner of data sources were available, including external to the aircraft. I have an attitude display on my iphone, ipads, and android phone, and all are better than that offered by any OEM airframer. My android phone gives accurate AH info for 48 hours, which is longer than my interest, but is certainly better than the 30 minutes that is certified by the OEM. If you are driving your shiny new A or B brand with 138 minutes, you already exceed your ISFD endurance by enough to be annoying, unless you have your string, glass, cat etc. At modest Mach, the current crop of nav charting on the ipad give enough info to fly level and nearly straight, slower is better, just like the early Garmin 3 pilot displays used to give a pseudo 6 pack. AOA is great to have, and I fly donuts every day, did in the military and do on corporate jets. HUD was nice, but all sensors can have a bad day, and a loss of electrical power will leave the argument back to simple subjects, your GPS, AOA, EIS/ECAM are giving info that look rather poorly without ergs. So, huff n' puff about how many angels can fit on the pin head, and then consider what you do when you cannot find the pin.. it all becomes moot. Any comprehension of correlation of performance, attitude, and any associated data source increases the likelihood that you can recognise a problem in the first place, and history shows that the problem isn't actually doing it, it is recognising that you need to do it. GPS (or other) ground speed is handy to have in mind at all times, it gives a nice idea of what sink rate to target on finals, it gives an idea of your wind component (think about stopping before going into rivers or oceans at the EOR.... like Mach number, it gives a nice idea of pitch changes needed to get a particular rate of climb or descent... or attitude adjustment to level off etc. If it helps, use it. if not, don't. The B777 AOA still generally gives a good backup to UAS cases, however TBC realised that there was an input from the ADC output that could give erroneous output. If you have an PFD/AI etc, and a VSI of any sort, (or an altimeter, GPS altitude... whatever, Cat, string, cup of soup in a glass...) then you can determine the difference between the effective flight path and the attitude, and thereafter, you don't need AOA at all, you have it from the AI. If that is accurate. The greater your comprehension of the data presented in your unique case, the higher likelihood that the outcome is a yawn not a headline making smoking hole in the ground. Due to some frustrating luck In the last 12 months, I have had over 2 dozen ADC failures, so pretty much take the position that understanding what is happening around you is paramount, not what the book demands. In the same period, at least 2 major hull losses have resulted from a single ADC failure, so some level of understanding is probably a good thing. If a loss of an AI/PFD, ASI or ALT is worrisome, then the time in cruise looking at the relationships of the data/performance needs to be ramped up. Cat/rope/cup, AOA, ATT/Pwr whatever it takes to float your boat. Note: the only AOA probes I have had fail are two on the B737... and a stuck one on a B747. They do fail, but so does eveything else on occasion |
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