ground speed
Join Date: Nov 2000
Location: Cambridge, England, EU
Posts: 3,443
Likes: 0
Received 1 Like
on
1 Post
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.)
Moderator
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!
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!
Last edited by Pilot DAR; 26th Dec 2011 at 14:14.
Join Date: May 2001
Location: 75N 16E
Age: 54
Posts: 4,729
Likes: 0
Received 0 Likes
on
0 Posts
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......
....It is great practice.
But GPS GS can give valuable clues in case of a blocked pitot......
is gps measured ground speed better than nothing in case of loss of ASI?
No ASI
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.
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.
Join Date: Dec 1999
Location: UK
Posts: 2,584
Likes: 0
Received 0 Likes
on
0 Posts
is gps measured ground speed better than nothing in case of loss of ASI
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.
Join Date: Apr 2009
Location: UK
Posts: 816
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
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!
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!
Last edited by bingofuel; 27th Dec 2011 at 20:42.
Join Date: Oct 1999
Location: UK
Posts: 3,325
Likes: 0
Received 0 Likes
on
0 Posts
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. 
Join Date: Feb 2001
Location: UK
Posts: 647
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Moderator
But "better than nothing", which is what the original poster asked? Of course.
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...
Join Date: May 2001
Location: 75N 16E
Age: 54
Posts: 4,729
Likes: 0
Received 0 Likes
on
0 Posts
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)...
I agree that Power + Attitude should alert you but it doesn't always (apprently)...
Join Date: Dec 2011
Posts: 2,460
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
a small change in the pitch attitude results in a fairly big change in the airspeed.
Ok, now tell me I fly the world's slowest draggiest aircraft anyway, so WTF do I know?
Last edited by Piper.Classique; 27th Dec 2011 at 21:32. Reason: thought about a few other things to say
Join Date: Dec 2011
Posts: 2,460
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Oct 2011
Location: UK
Age: 40
Posts: 211
Likes: 0
Received 0 Likes
on
0 Posts
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
(I jest...)
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
(I jest...)
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.
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.
Thread Starter
Join Date: Dec 2009
Location: moon
Posts: 80
Likes: 0
Received 0 Likes
on
0 Posts
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,
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,
Last edited by rapidshot; 28th Dec 2011 at 04:42.