ground speed

[rapidshot]

is gps measured ground speed better than nothing in case of loss of ASI?

[Genghis the Engineer]

For navigation it's fine.

For any other form of flight guidance, no, it can be dangerously misleading. Fly by visual pitch attitude and power.

G

[Gertrude the Wombat]

Pitch and power works. If you haven't flown a circuit and approach and landing with the ASI covered up give this a try with an instructor. (Maybe not into the shortest runway you can cope with normally, you will naturally feel inclined to err on the side of coming in a bit fast.)

[Pilot DAR]

To support Genghis and Gertrude, we generally use indicated airspeed (IAS) for two things in flight, and they are surprisingly unrelated.

The first, where GPS is a great substitute, is obviously "when will I get there?". Use GPS for this. The IAS is affected by numerous factors in cruise flight (wind being the greatest) that GPS is superior anyway.

The second is the relationship between the air passing over the wing relative to the air required to pass over the wing for the given flight condition. ASI is pretty good for this, up to a certain pitch attitude, though even after that it's giving information which will help you to stay safe. GPS is not at all good for this, the GPS speed does not consider the affect of wind over the wing, and the information can be harder to interpret quickly, relative to the rate at which the airspeed can change in those conditions of flight. The GPS will, also not be positioned in your scan, where the ASI is required to be seen, so you'll be searching the panel for it when you should be looking out the windshield.

Interestingly, I was flying the Tiger Moth a few weeks back The rear cockpit (where I was flying from) ASI was actually annoyingly close to me to see easily, and it's antiquated markings difficult to quickly interpret. The front cockpit (no one up there in my way) ASI was easier to see, but I think out of calibration by quite a lot. The wingstrut mounted spring on was there, and seemed to work well, but ultimately, I just flew by feel, and that worked best. Now I was not going on a cross country, so navigation was not a factor.

In the same period, I was also flight testing a Siai Marchetti 1019A. For a rather advanced turbine powered aircraft, which has fairly high wing loading when heavy, I was amazed to figure out that it has no stall warning system at all, and only a three knot margin between the first indication of a stall, and the break. It also had very unusual pitch forces near the stall (sometimes reversing = very poor "feel") This drove me to watch the IAS fairly carefully on approach and departure.

For the certified GA aircraft, any can be happily landed without any reference to the ASI, with a bit of practice. I highly recommend this be done by every pilot recurrently - it's going to happen to you sometime!

http://i381.photobucket.com/albums/oo252/PilotDAR/Aircraft/IMG_0826.jpg

[englishal]

When I did my commercial my FI made me depart the circuit, fly around in various phases of flight then rejoin and land with nothing but oil temp and oil pressure
....It is great practice.

But GPS GS can give valuable clues in case of a blocked pitot......

[Whopity]

is gps measured ground speed better than nothing in case of loss of ASI?No you are better off with Attitude and Power. I recently flew an approach with no ASI and afterwards looked at the GPS in my phone to see what speed I had touchdown at. I was within 2 kts of the target speed.

[cessnapete]

Flew a C182 back from Italy VFR, recently. No ASI readings, but no problem.
One panel mounted GPS and one stby hand held. Always had an idea of surface windspeed from Volmet and ATC chat, so easy to work out appropriate GS required for various stages of a flight.
On return to UK, maintenance organisation cleaned out insect debris lodged in pitot feed in windsceen sidepost area.

[Agaricus bisporus]

is gps measured ground speed better than nothing in case of loss of ASI

This being PPRuNe many people haven't taken the trouble to either read or understand the question but once you have done so it is self-evident that the answer must be "Of course it is".
Anything is better than nothing. Sticking your hand out of the window and feeling the air is better than nothing.
Of course there are caveats as gven above and GPS may well not be the best way to handle loss of ASI but that isn't the question asked, is it?

But "better than nothing", which is what the original poster asked? Of course.

[Torque Tonight]

Ag, you had the same thought process as me. Of course it's better than nothing. Not much better, but still better.

Had an ASI failure recently when the pitot air pipe disconnected behind the instrument panel in flight. Fortunately, my aeroplane is one that can be flown seat of the pants and by feel. Stick forces give very good feedback of airspeed and ultimately the incipient and full stall are very clear from buffet and other clues. Flying the approach at normal attitudes and powers and experiencing familiar stick forces and motion of the airframe resulted in a safe landing. No big deal.

[bingofuel]

Whopity, I take it there was no wind so that your GPS ground speed matched your Target IAS?

But I agree power and attitude are the primary references, although to try to answer the original question, is GPS better than nothing? well if you can get a wind speed and direction from ATC then you could calculate a rough target ground speed from which to fly an approximate approach speed, but I would not rely on it very much!

[Shaggy Sheep Driver]

I've just returned from a session conducting tours on our Concorde. The ASI on that aeroplane maxes out at about 550 knots. Conc cruised at M2, about 1,170kts over the ground. But you'd see about 435kts IAS at 60,000' and M2. :O

[chrisN]

Not to detract in any way from above advice about knowing power and attitude settings, and agreeing that one should be able to fly sufficiently accurately without ASI or most other instruments.


But nevertheless, if you fly into wind (of course, all competent pilots know how to determine that direction if the wind is significant), and note GPS speed; then downwind at the same airspeed/settings; and note the new GPS speed; the difference between the two is twice the wind speed at that altitude, and GPS speed into wind plus wind speed is your true airspeed. At close to ISA, that is close to IAS, otherwise corrections need to be allowed for (beyond my capability for mental arithmetic as I don’t know the correction factors off by heart!).

Trying it might make a useful exercise sometime when you have nothing better to do while bimbling about.


IMHO.


Chris N.

[Pilot DAR]

But "better than nothing", which is what the original poster asked? Of course.

I'm not rushing to agree that GPS groundspeed information would be better than nothing for certain phases of flight. In cruise navigation, sure... In close final approach, I would not agree.

GPS speed information might not be easily perceived with a quick glance, as ASI indications are. (Ever flown a plane where the ASI turns the other direction, or zero is not at the top?) So, if you take your attention away from flying the approach, to look for the unfamiliar, difficult to perceive, GPS groundspeed, and then do the mental math to factor out the wind speed, I ecxpect that you're much further behind, and distracted than you would have been if you had simply continued to fly the approach with what you knew you had.

This is analogous to trying to restart a failed engine while going down final approach. Too distracting, just make a good job of the approach. A stable approach is where things are not changing at a rate which exceeds the pilot's capacity = the pilot is "ahead" of the plane. A pilot probably puts themselves back behind the plane if trying to resolve the GPS into useful IAS for an approach. The same pilot could fly a very stable approach without the the ASI, and the distraction of the GPS information.

We did do it before GPS! Sometimes technology can be more distracting than helpful...

[englishal]

I quick cross reference to GPS speed might lead you to quickly diagnose a blocked pitot tube if you're climbing. This has caught out many PPLs and even several airliners in the past. If you ASI says 130 kts and you are aiming for 100 kts and you don't spot that at this attitude it should read 70 kts, then a quick glance at the GPS which might show a slow GS could make one realise there is an issue.

I agree that Power + Attitude should alert you but it doesn't always (apprently)...

[peterh337]

Just don't expect to be doing a "max performance landing" if using power+attitude alone, because a small change in the pitch attitude results in a fairly big change in the airspeed.

Also it is not that easy to read off the pitch on the normal type of horizon. On the KI-256 I can barely tell the difference between say 4 and 6 degrees.

With a blocked pitot I would head for a nice long runway.

[Piper.Classique]

a small change in the pitch attitude results in a fairly big change in the airspeed.depends on the aircraft. Clean, slippery gliders and draggy trainers are very different beasties. Hang out the flaps or the airbrakes and things change, naturally. BUT if you know the machine at all well you should be able to fly the attitude. Hell, that's the basis of visual flying anyway. GPS will be handy for the nav, of course, but I wouldn't care to do the maths on approach.The feel of the aircraft should be telling you something unless you are flying in boxing gloves. Mostly the runway at any airfield used for training will be plenty long enough to land a light single even if you are a touch fast on final and float out the landing. Unless you routinely add a few knots for the wife and kids, plus some more for the full fuel and a few knots more for the wind on the nose, and get used to a grossly overfast final anyway.......I wouldn't be using an AH for this, just the one I see out of the window. It's bigger and better.

Ok, now tell me I fly the world's slowest draggiest aircraft anyway, so WTF do I know? ;)

[peterh337]

I think that if I lost my pitot I would trim for level flight at the desired final approach IAS and aircraft config, and use the GPS to determine that trim setting (by flying two reciprocal headings and taking the average GS).

Then, the transition from level flight to the final approach would be just a power reduction, whose magnitude is simply according to the required ROD.

[GeeWhizz]

Essentially I have 3 speeds: climb (including take off and go around I.e. full power), cruise, and approach. Each of these can be set quite accurately by rpm/MAP gauges, possibly even to within 3-5kts in cases of Vfe. This of course relies on having at least 1 or 2 hours on type.

GS is almost pointless for the visual approach and landing, but obviously much more relevant for navigation. Finding GS using good old fashioned timed distances will do. And actually from the timed distance and cruise rpm/MAP setting, a headwind/tailwind is easily calculated. Couple it with the applied drift calculation and you have a good enough wind direction also. This could take 15 seconds or so maybe?

I think these calculations are so basic that in the cruise a GPS GS may as well be used as the primary rwference. One would quickly notice if it was unacceptably inaccurate and revert to the proper methods of flying :D (I jest...;))

[abgd]

Well, how big an error in airspeed would be significant? If you fly an approach at 70 kts and that's 1.3 x the stall speed, then an error of 25 knots could be catastrophic. Can the wind at circuit height exceed 25 knots? Sure... Perhaps quite a lot of the time, depending on where you fly.

On my first solo I somehow checked and set the auxillary static port open and the airspeed indicator over-read by 10 knots, so I was flying my approaches way too slowly. I really didn't like the feel of the aircraft, so I added an extra 5 knots for good luck and asked my instructor to take a look later.

So, if my airspeed indicator were to fail and I were to use the GPS groundspeed measurement to fly by the numbers I figure I would crash quite a high proportion of the time, I feel confident that I could land a PA38 without instruments - at least on a longish runway where a little extra airspeed would do no harm.

I see that you can calculate windspeed by flying a circuit - but this will only be the windspeed at circuit height. By the time you get to final approach the wind may be doing something very different. However, I can see that it might be useful for setting the trim. And for navigation with a GPS you shouldn't really need to know your airspeed - just groundspeed, position and endurance, which you can work out from rpm.

So using a GPS to guesstimate airspeeds could prove much worse than nothing, at least for landing and take-off.

[rapidshot]

I think attitude flying is the best thing to do with rare scanning of GS when navigating, thats where i came to, flying in various headings is also a good tool to determine the wind speed but i am never sure that wind flow is so steady as in the text books,

As for landing if there is tower, attitude plus GS both may be used, Without tower i agree with attitude flying only,shortly this is the summary i extracted for myself,

There is one more question itching my brain,
What about Va and Vfe in case of ASI loss, apart from that I still read ongoing discussions whether they are TAS or IAS,

[Big Pistons Forever]

Every pilot should be able to do a circuit with the ASI covered. It is no big deal and I often use this technique with more advanced training for pilots who have gotten into the bad habit of chasing the airspeed :mad:

There really should be no reason to look at the GPS if you know the basic power settings and pitch attitudes for the normal phases of flight.

[rapidshot]

Big Pistons, i very well agree with you but within years as hours built up that training occupies a small fraction of hours flown, everyone gets used to scan those instruments,
i bet everyone will remember the most famous sentence , "watch your speed first", so i dont think some basic skills will develop under these circumstances or there was something wrong with the way i was trained,

Also there is a concept i first met almost a year ago, i really wish to learn what you all think about attitude flying using "bar width",

[abgd]

As for landing if there is tower, attitude plus GS both may be used, Without tower i agree with attitude flying only,shortly this is the summary i extracted for myself,

I don't really understand what you have in mind there - perhaps that the tower will give you the airspeed at ground level, so you can compensate?

I'm still don't think I'd go with that - winds can change quite quickly, or even blow in opposite directions at each end of the runway.

[rapidshot]

I don't really understand what you have in mind there - perhaps that the tower will give you the airspeed at ground level, so you can compensate?

that is exactly what i think, my opinion is if the wind changes quickly any method should take you in trouble without ASI,

[Pilot DAR]

the most famous sentence , "watch your speed first", so i dont think some basic skills will develop under these circumstances

It is true that too much concentration on instruments will distract a pilot away from "feeling" the plane during maneuvering. Any single engine certified GA type will give the pilot ample feel, as to how it is flying, and what it's speed is relative to the stall. The idea that a slow to refresh, uncorrected GPS groundspeed has any use to replace an ASI for maneuvering type flying is silly.

This, to me, speaks of the trend away from learning to "fly" a plane, toward learning to read what its indicators are telling you - because the computer flight simulators are demonstrating that. It's easy for the computer to provide lots of data, and decent visuals. Its difficult for appropriate force feedback to be provided by the computer, and impossible to simulate G forces.

The interpretation of all of these cues in combination is what it takes to fly well. Yes, you can still fly with a cue or two missing, but it will be less well.

It's nice to have all of this extra information when flying, and sure, I use it too, but it is a vital basic flying skill to be able to feel what the plane is telling you, in the absence of indicated numbers. Ths is particularly important when you're flying something different, and you have not memorized all the speeds anyway! (Some planes don't have coloured bands on the ASI to remind you!).

You're paying lots to be off the ground, why would you want to give up the only means to actually hone your skills of being "at one" with the feel of the plane, to allow yourself to concentrate on a bunch of indicators which you can also use on your computer at home? Presuming that you're flying in good visibility, and in airspace which does not demand strict adherence to altitudes, Try flying around with no reference at all to any indicator, other than to monitor engine condition an fuel from time to time. When you're realizing how much the plane is telling you without your reading any numbers, try some slow flight that way. Then, go and land that way.

Your slow flight practice will have reminded you of the feel of just getting on the back side of the power curve, and how to just get there, without going too slowly. As long as you can feel that, without the crutch of an indicator telling you, you can safely land the plane in most day VFR conditions.

You'll surprise yourself. And, you'll be that much better prepared to fly safely when something does fail for real! These skills should be practiced regularly

[rapidshot]

Actually my question was not about improving the basic skills but related to, say as an example, speeds under vfr into imc conditions, my fault if this topic turned into a discussion about developing skills,
Since English is not my native language may be i couldnt express myself but my question in one of my replies on Va and Vfe to me is self explanatory, are they ias or tas, i really want to learn and discuss,

[Pilot DAR]

if this topic turned into a discussion about developing skills, Don't worry Rapidshot, that's what the forum is for...

"V" speeds will be an indicated airspeed (IAS), and related to calibrated airspeed (CAS), which is a corrected IAS anyway. TAS and groundspeed do not enter into it, as the V speed is the affect of the actual air on the aircraft. The aircraft does not care about it's groundspeed (until it touches the ground, but even then, it's only the tires), and for the purposes of the affect of air on the airframe, it does not care about it's TAS.

Of course speeds which are limiting, and expressed as "V" speeds (Vne, Va, and Vfe, for example) must not be exceeded. The aircraft is designed so it is easy, while being careful, to not accidentally exceed these speeds.

You're not going to accidentally exceed Vne during controlled flight, unless you're oblivious to the howling sound of the air, and the whining of the engine at near red line RPM, and an unwinding altimeter. If you've buggered up aerobatics, you could - so obviously, aerobatics with a U/S ASI is very foolish.

If you think you're near or above Va, avoid really rough air, and abrupt control movements until you've slowed. When the controls begin to feel more mushy, you're below Va, and safe to do what you have to do.

In a GA aircraft, there is no need to be pushing Vfe, so just make sure you're nice and slow, before extending flaps, and you'll be fine. During certification testing, Vfe will have been established with a reasonable margin to allow for pilot technique.

Because all the "V" speeds are IAS, using the GPS to adhere to them is troublesome. If you're maneuvering the aircraft and getting close to one of these limits, you're probably too busy to be working out the factors between IAS and GPS groundspeed,

[rapidshot]

May i ask the reason unwinding altimeter

And last question, if we enter little gusty air, it is obvious that we are not able to know that what is waiting us next, may be a gust at the limits of the envelope, so do you think that we must reduce the speed near Va if we meet such a humble gust,

Edit: and i wonder why Vfe is given as IAS, as we know the inherent dynamic characteristics of the structure such as natural frequencies are independent of the density of the air, but the excitation forces are probably a function of density, so i dont think there is a simple relation like the relation between Tas and Ias when flutter is considered, i really wonder the reason, and i think there is no time to think all of those and make some calculations when the airplane starts to dive,

[peterh337]

I reckon flutter is related more to TAS (the actual airflow velocity) than IAS, but the ASI has to be marked in IAS.

This is why, AIUI, when some aircraft types are modified with a more powerful engine which enables them to climb higher (where the TAS gain over IAS is greater) the Vne figure is reduced.

[Shaggy Sheep Driver]

Hey guys - the airframe doesn't give a fig for TAS. It only feels IAS (and Mach).

[rapidshot]

Hey guys - the airframe doesn't give a fig for TAS. It only feels IAS (and Mach).

This is true only at the sea level without compresibility effects, i guess, lol , your statement will start a new discussion around Eas,

[Shaggy Sheep Driver]

Nope. Think of 'coffin corner'. Consider that all airframe limits are IAS (or Mach) regardless of height, altitude, or FL.

VFE is the same regardless. If your aeroplane is going to stall at 100 knots indicated, it will stall at 100 knots indicated, at sea level or at 40,000'.

[rapidshot]

Depends i think, the designers limit the speeds with the most conservative case , probably that case occurs at sea level with MTOW and they mark that on the Ais,

As for stall speeds obviously it is same at all altitudes due to the logic behind the design of pitot tube, the density is just cancelled out of the equations

[Shaggy Sheep Driver]

No, it depends on aerodynamics. The ASI WILL read 100 kts at the stall in my example above, regardless of altitude (until mach effects come in at very high altitudes and we get mach stall ahead of AoA stall). Doing aeros at several thousand feet would be wierd if all your entry and limit speeds (on the ASI of course) were different to doing them on the deck at an airshow!

You'll be telling me next you get a higher IAS when you turn upwind to downwind!

[rapidshot]

Stall speed is just a measurement, as i wrote before the density has no effect , but as for the Va and Vfe i dont think this holds, to me most critical cases are put in the Poh and the Asi

[Shaggy Sheep Driver]

I hope you are not a licenced pilot, rapid!

Stall isn't rally a speed, it's an AoA. But for an given configuration and weight etc the stall AoA will be reached at a particular indicated airspeed (the point at which the airflow breaks away from the wing's surface). This IAS is always the same for that config etc, though it will occur at vastly different TASs if the air density varies.

[rapidshot]

You'll be telling me next you get a higher IAS when you turn upwind to downwind!

No Sir, i will never tell that, even think of it ;)

[rapidshot]

I hope you are not a licenced pilot, rapid!,
I am not a pilot, i just hold a pilot license

Stall isn't rally a speed, it an AoA. But for an given configuration and weight etc the stall AoA will be reached at a particular indicated airspeed (the point at which the airflow breaks away from the wing's surface). This IAS is always the same, though it can occur at vastly different TASs.

:) lets better stop here, i bet we both do not want to find ourselves discussing the solutions of Navier Stokes equations of the boundary layer under this topic, stall is said to be independent of speed but i have always wondered how Reynolds Number affects the max AoA if this is true :O

I am sure that i Have never spelled the word stall here in this threat before

[englishal]

Briefly.........Vne (and what the airframe "feels") is indeed determined by TAS and not IAS. Vne actually decreases with altitude and if you hot rodded your engine and took your plane up high, you could actually encounte Vne (and flutter) below the Vne marked on the ASI. I guess it is marked on the ASI for a sealevel IAS, because they know that up high the aeroplane as designed would never (in normal flight) reach a TAS which would exceed Vne (due to power limitations of the engine etc). All bets are off if you decided to tinker with your engine and add a Turbocharger (for example) without any further modifications to the airframe.

It is to do with the speed the molecules of air pass over the control surfaces and not the density of the air which will determine when flutter will occur (the reason for Vne).

Here, this explains it better than I can ;)

http://www.vansaircraft.com/pdf/hp_limts.pdf