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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 25th May 2019, 11:08
  #21 (permalink)  
 
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Originally Posted by Capn Bloggs View Post
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!
Sure it will be decresing. But the aircraft will start descending at a safe speed/AoA to keep the wing flying. As you are at FL370, no problem with terrain clearance, and the ATC will get the other traffic out of your way. Plenty of time to have a look at the QRH, which is not that difficult to find anyway, as it's always the first thing under the Performance Inflight tab.

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 View Post
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...
Now, to be fair, the ET302 did not apply UAS memory items (not suggesting they did a mistake, as it was a tremendously difficult situation and remains to be investigated). MCAS activated with flaps up, so should they have elected to perform UAS memory items, the pitch/thrust would be 4deg pitch.75% N1.

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.
Last time I checked there is a significant thrust difference between 75% N1 and 94% N1.

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.
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Old 25th May 2019, 13:01
  #22 (permalink)  
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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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Old 25th May 2019, 14:43
  #23 (permalink)  
 
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Pilot DAR, Tech Log?
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Old 25th May 2019, 15:00
  #24 (permalink)  
 
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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
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Old 25th May 2019, 15:27
  #25 (permalink)  
 
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Originally Posted by YYZjim View Post
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
........hmm. Might draw some comments with that one.
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Old 25th May 2019, 15:37
  #26 (permalink)  
 
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Originally Posted by F-16GUY View Post
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.
Agree, but I do think if the pilots on AF447 had looked at their groundspeed (I think it went down to 107 kts towards the end) it might have rung a bell?
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Old 25th May 2019, 16:29
  #27 (permalink)  

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Originally Posted by Capn Bloggs View Post

Are you a 737 pilot? Do you know the difference?
No, I'm not a 737 pilot. The subject title doesn't say the discussion is exclusively about the B737.

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.

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Old 25th May 2019, 18:56
  #28 (permalink)  
 
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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.
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Old 25th May 2019, 19:04
  #29 (permalink)  
 
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GS is a good tool in the box.
Can be used for reference, but in no way should it take precedence over pitch/power.
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Old 25th May 2019, 22:32
  #30 (permalink)  
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Originally Posted by Small cog View Post


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.
OK. Having started this thread- it’s an interesting one, let me try a reply. First we are talking big planes here and no visual reference. And of course at the time of IAS failure you know your groundspeed. As stated earlier this doesn’t apply to a situation akin to a blindfold pilot , at an unknown altitude, airspeed or wind suddenly being unblindfolded, with no instruments. He is in stable flight prior to the failure. He knows the current wind and GS and also has accurate winds for all altitude on the flight plan. Within 5
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

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Old 25th May 2019, 22:43
  #31 (permalink)  
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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


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Old 26th May 2019, 01:48
  #32 (permalink)  
 
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Originally Posted by yanrair View Post
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.
Because the thrust required would’ve been more like 95% N1 than 70%, and because any changes in the wind would have you chasing the GS in a way that could be quite detrimental. Still better than what happened, though!
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Old 26th May 2019, 01:53
  #33 (permalink)  
 
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A330-300 normal cruise is 2.5 degrees pitch and 78% N1

GPS Altitude and GS is very handy BUT use carefully the GS....
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Old 26th May 2019, 02:24
  #34 (permalink)  
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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...

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Old 26th May 2019, 02:37
  #35 (permalink)  
 
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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.....
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Old 26th May 2019, 03:50
  #36 (permalink)  
 
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A330-300 normal cruise is 2.5 degrees pitch and 78% N1
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?
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Old 26th May 2019, 04:08
  #37 (permalink)  

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? Are you asking about the angle of incidence between the wing chord and fuselage longitudinal axis?
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Old 26th May 2019, 04:22
  #38 (permalink)  
 
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A330-300 normal cruise is 2.5 degrees pitch and 78% N1
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?
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Old 26th May 2019, 04:59
  #39 (permalink)  
 
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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.
Let's not get carried away. I never said use GPS GS to the exclusion of everything else. Thanks for the lecture on TAS/Wind/Altitude. I didn't know that.
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Old 26th May 2019, 05:25
  #40 (permalink)  
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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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