Groundspeed without GPS

[indian airlines]

If an airplane does not have a GPS, is there any other way it can get a groundspeed reading, or is all it gets an indicated airspeed reading?

[screaming_emu]

yup, its pretty easy too. When you plan a flight, you plan it using set checkpoints on the ground. Beforehand, you figure out how far apart those checkpoints are. Once you pass over the first one, you start a stopwatch. Once you reach the next one, you stop the watch. Now that you know how far between the points and how long it took you to fly that distance, you can figure out your groundspeed. We do this with this flight "computer" (more like a sliderule)...now back to my studying.

[indian airlines]

yup, its pretty easy too. When you plan a flight, you plan it using set checkpoints on the ground. Beforehand, you figure out how far apart those checkpoints are. Once you pass over the first one, you start a stopwatch. Once you reach the next one, you stop the watch. Now that you know how far between the points and how long it took you to fly that distance, you can figure out your groundspeed. We do this with this flight "computer" (more like a sliderule)...now back to my studying.

Makes sense. Nowadays is groundspeed usually calculated from the airspeed or vice-versa, if you know what your headwind or tailwind is? Or does it not matter. Is it more accurate done one way or the other?

[screaming_emu]

Makes sense. Nowadays is groundspeed usually calculated from the airspeed or vice-versa, if you know what your headwind or tailwind is? Or does it not matter. Is it more accurate done one way or the other?

Its a lot more accurate to calculate it using the distance and time. The reasons for this are that a) The ony weather product dealing with winds aloft is simply a forecast. Sometimes they're right, sometimes they aren't. b) Wind changes with altitude. While you can take the forecasted wind for two altitudes and interpolate between them, there's no gaurentee that is going to be right. But with fairly exact calculations that you have made (within about half a mile for the distances, and a couple seconds for the time) you should be able to come up with a pretty good number.

Some of the more advanced avionics setups (mostly the glass cockpits) will actually compare your true airspeed, heading, ground track, and ground speed to figure out what the wind is. These usually put a little arrow as well as telling you the direction the wind is coming from and its strength. Pretty nifty.

[indian airlines]

Suppose you're about to land, and you know there's a 20kt tailwind. Your airspeed reads 120 (assume that's the landing speed for the plane we're talking about - I have a question here too, when they say landing speed, do they mean groundspeed or airspeed?), but since you have a 20kt tailwind, your groundspeed is 140kts. Do you need to be concerned that you're flying 20kts too fast?

[screaming_emu]

Suppose you're about to land, and you know there's a 20kt tailwind. Your airspeed reads 120 (assume that's the landing speed for the plane we're talking about - I have a question here too, when they say landing speed, do they mean groundspeed or airspeed?), but since you have a 20kt tailwind, your groundspeed is 140kts. Do you need to be concerned that you're flying 20kts too fast?

When they say landing speed is 120, its going to be 120 kts of airspeed. Think of it this way. If it was 120 kts of groundspeed, but you had a 120 kt tailwind (extremely exagerated, but it will help prove the point), what would the airplane's airspeed be?

You are correct that with an airspeed of 120 kts, and a tailwind of 20 kts, your groundspeed is 140 kts. While it isn't really going to change the way the airplane is flying, it will affect your landing considerably. Because of the increased groundspeed, your landing roll is going to be quite a bit longer. Also in bigger airplanes, landing at such a high groundspeed may get you near your max tire speeds. That is why whenever possible, its best to land with a headwind. Not only will it decrease the speed which your wheels have to accelerate to when you touch down, but it can decrease your landing roll by quite a bit. It also gives you more time to think about the landing because stuff is coming at you slower.

[indian airlines]

Awesome, thanks. I get it now.

[ptbodale]

Excellent EMU. Many moons ago when I learned to fly we did it your way. GPS wasn't around in the 70's.

[screaming_emu]

Excellent EMU. Many moons ago when I learned to fly we did it your way. GPS wasn't around in the 70's.

yeah, everybody is still taught that way, even though the GPS is waaaay easier. I can attest that its so easy to rely on the GPS for everything. I get the feeling that some people once they graduate from here, once they go to fly something without a moving map they're completely lost.

[indian airlines]

Slightly related question...suppose you did infact have a 120 kt tailwind on a plane whose landing speed was 120 kts. This efffectively makes its groundspeed 0kts right? So what happens when the plane touches down? You would expect it to stop instantly right, since it has a groundspeed of 0kts, but wouldn't the tailwind affect it when it's on the ground too?

[MaxPower]

Hey Guys, This is one of the most interesting thread I have ever read !Thanks to Tanuj of his curiousity and Joe knowledge on that issue !

Thanks guys ! Liked reading your posts !

[screaming_emu]

Slightly related question...suppose you did infact have a 120 kt tailwind on a plane whose landing speed was 120 kts. This efffectively makes its groundspeed 0kts right? So what happens when the plane touches down? You would expect it to stop instantly right, since it has a groundspeed of 0kts, but wouldn't the tailwind affect it when it's on the ground too?

Well, with a 120 kt tailwind, you would actually have a groundspeed of 240.

Look at it this way, When you have a headwind, the way you would figure out a groundspeed is Airspeed-headwind component. If it is a tailwind, its Airspeed+tailwind.

Now if you had a 120 kt headwind, and a current airspeed of 120 kts, then you'll actually just be staying in place. Give it a shot on flight sim, use a plane like the 172 and set the winds so that they're about 55-60 kts pointing straight down the runway. Or if you want to use something bigger, such as the 747, just set the windspeed to something around 180-200. Pretty interesting trying to land like that :-)

[indian airlines]

Well, with a 120 kt tailwind, you would actually have a groundspeed of 240.

Look at it this way, When you have a headwind, the way you would figure out a groundspeed is Airspeed-headwind component. If it is a tailwind, its Airspeed+tailwind.

Now if you had a 120 kt headwind, and a current airspeed of 120 kts, then you'll actually just be staying in place. Give it a shot on flight sim, use a plane like the 172 and set the winds so that they're about 55-60 kts pointing straight down the runway. Or if you want to use something bigger, such as the 747, just set the windspeed to something around 180-200. Pretty interesting trying to land like that :-)

Yeah, I'll try that.

I should have asked my previous question a little better...what I mean is, if you have a 120kt tailwind on a plane with a specified 120kt landing speed, the plane would have to set its own speed (i.e. velocity input from the plane) to 0, so that the airspeed would be 120kts. Right? Or am I just confusing myself more.

So if that is right then, when the plane touches down, it has no "speed" of its own - what happens then?

[screaming_emu]

Yeah, I'll try that.

I should have asked my previous question a little better...what I mean is, if you have a 120kt tailwind on a plane with a specified 120kt landing speed, the plane would have to set its own speed (i.e. velocity input from the plane) to 0, so that the airspeed would be 120kts. Right? Or am I just confusing myself more.

So if that is right then, when the plane touches down, it has no "speed" of its own - what happens then?

I think you have it backwards. Wind is always described in the direction its coming from. Like if you have a westerly wind, its coming from the west, if you have a headwind, its coming from in front of the pane, a tailwind is coming from the rear of the airplane.

But to answer your question, its like a boat in a stream. Say you were in a helicopter. You're hovering so that your airspeed stays zero. The wind is going to pick you up and push you at the same speed it is goin. Just like if you're in a kayak in a stream, you dont have to row, the stream will pull you along. But if you're goin upstream, if you want to keep the same speed relative to the shore (instead of the water) you're going to have to work twice as hard.

[Shadower]

So in theory you can make a plane “hover” in place if you have a wind speed greater than the stall speed (or whatever the term is) of the aircraft.

Would be kinda neat to see that :D

[screaming_emu]

Yup, I've never done it, but we've had people here fly backwards before. One thing we practice is "slow fllight". We slow the airplane down to the slowest possible airspeed to get used to maneuvering it at slow airspeeds. This is probably about 50 kts. If its windy enough, and you climb high enough that the winds are stronger than that, you can start goin backwards.

[AJ]

At Qantas we use a method for avoiding shear called 'Reference Ground Speed'. IAS is adjusted to ensure that any change in wind before landing can be easily counteracted.

You start with your Vref, say 130kias. Adjusting for temperature difference from ISA we add or subtract 1 knot for every 5 degree celcius. Say landing in Melbourne (400' amsl) on a 30 degree day, ISA = 14 therefore the RGS adjustment is +3 knots. Adjusting for altitude by 1 knot you basically have a TAS for final, 130 +3 +1 = 134 knots.

Now this needs to be adjusted for the surface wind. Say landing in a 10 knot headwind the required groundspeed at the threshhold will be 124 knots. Due to the momentum of a heavy aircraft if the groundspeed down final is maintained at 124 any low level change in wind will be counteracted without IAS falling below Vref and no power change. So if a 20 knot headwind is blowing down to 100' the approach will be flown at 144kias for a groundspeed of 124! It works a treat.

The limitation is that the aircraft cannot cross the threshhold at more than Vref+20 as landing on the nosewheel becomes an issue!

Now only two of our 767s have GPS so the groundspeed readout is provided by our three Inertial Reference Systems (IRS), which are also suprisingly accurate to within a knot or two. As most pilots use the same margin of error it is not an issue!

Flying into Melbourne with the notorious low level northerly jet I've had reference groundspeeds in the region of 90 knots!

[Van Hoolio]

One of our Cessna 172s has DME, and the DME unit will calculate your groundspeed and time to station, which I imagine is reasonably accurate if you are tracking directly to or from the station.



1-1-7. Distance Measuring Equipment (DME)

a. In the operation of DME, paired pulses at a specific spacing are sent out from the aircraft (this is the interrogation) and are received at the ground station. The ground station (transponder) then transmits paired pulses back to the aircraft at the same pulse spacing but on a different frequency. The time required for the round trip of this signal exchange is measured in the airborne DME unit and is translated into distance (nautical miles) from the aircraft to the ground station.

b. Operating on the line-of-sight principle, DME furnishes distance information with a very high degree of accuracy. Reliable signals may be received at distances up to 199 NM at line-of-sight altitude with an accuracy of better than 1/2 mile or 3 percent of the distance, whichever is greater. Distance information received from DME equipment is SLANT RANGE distance and not actual horizontal distance.

c. Operating frequency range of a DME according to ICAO Annex 10 is from 960 MHz to 1215 MHz. Aircraft equipped with TACAN equipment will receive distance information from a VORTAC automatically, while aircraft equipped with VOR must have a separate DME airborne unit.

d. VOR/DME, VORTAC, Instrument Landing System (ILS)/DME, and localizer (LOC)/DME navigation facilities established by the FAA provide course and distance information from collocated components under a frequency pairing plan. Aircraft receiving equipment which provides for automatic DME selection assures reception of azimuth and distance information from a common source when designated VOR/DME, VORTAC, ILS/DME, and LOC/DME are selected.



e. Due to the limited number of available frequencies, assignment of paired frequencies is required for certain military noncollocated VOR and TACAN facilities which serve the same area but which may be separated by distances up to a few miles.

f. VOR/DME, VORTAC, ILS/DME, and LOC/DME facilities are identified by synchronized identifications which are transmitted on a time share basis. The VOR or localizer portion of the facility is identified by a coded tone modulated at 1020 Hz or a combination of code and voice. The TACAN or DME is identified by a coded tone modulated at 1350 Hz. The DME or TACAN coded identification is transmitted one time for each three or four times that the VOR or localizer coded identification is transmitted. When either the VOR or the DME is inoperative, it is important to recognize which identifier is retained for the operative facility. A single coded identification with a repetition interval of approximately 30 seconds indicates that the DME is operative.

g. Aircraft equipment which provides for automatic DME selection assures reception of azimuth and distance information from a common source when designated VOR/DME, VORTAC and ILS/DME navigation facilities are selected. Pilots are cautioned to disregard any distance displays from automatically selected DME equipment when VOR or ILS facilities, which do not have the DME feature installed, are being used for position determination.

[indian airlines]

Well, with a 120 kt tailwind, you would actually have a groundspeed of 240.

Look at it this way, When you have a headwind, the way you would figure out a groundspeed is Airspeed-headwind component. If it is a tailwind, its Airspeed+tailwind.

Now if you had a 120 kt headwind, and a current airspeed of 120 kts, then you'll actually just be staying in place. Give it a shot on flight sim, use a plane like the 172 and set the winds so that they're about 55-60 kts pointing straight down the runway. Or if you want to use something bigger, such as the 747, just set the windspeed to something around 180-200. Pretty interesting trying to land like that :-)

This is supercool...I don't know if it would happen like this in real life, but I tried it, and atleast in flightsim, with a 200 kt headwind, it's possible to make the 777 completely hover in one spot. At the time of hover, the IAS reads 190 kts and groundspeed reads 2 kts.

EDIT: Not only does it hover, but it then starts moving backwards, though once it starts moving backwards it starts falling out of the sky.

EDIT 2: It was able to touchdown like a helicopter...sort of hover in one spot and slowly descend without changing horizontal position. The moment it touched down, it started moving backwards on the runway, but the IAS still read somewhere near 180-190, though slowing down all the time. Kinda confusing, but extremely cool.

Could this stuff happen in reality, if the headwinds and stuff were set up as above?

[screaming_emu]

Could this stuff happen in reality, if the headwinds and stuff were set up as above?

sure, to fly all an airplane needs is the sufficient airspeed. :-)

The reason you started drifting backwards is cause the plane was slowly accelerating with the wind. To stay in one place you'll have to keep the airspeed constant using power.

[bbuse]

Yup, I've never done it, but we've had people here fly backwards before. One thing we practice is "slow fllight". We slow the airplane down to the slowest possible airspeed to get used to maneuvering it at slow airspeeds. This is probably about 50 kts. If its windy enough, and you climb high enough that the winds are stronger than that, you can start goin backwards.
I have heard of people flying backwards like this in a slower plane such as a Cessna 150. I could also see this being possible in something like a Piper Cub.

[chrisburns]

I have heard of people flying backwards like this in a slower plane such as a Cessna 150. I could also see this being possible in something like a Piper Cub.


I almost got it going backwards, we were going 2 knots!!! A car going maybe 25 blew right by us, it was pretty sweet and fun.

[dave_333]

I almost got it going backwards, we were going 2 knots!!! A car going maybe 25 blew right by us, it was pretty sweet and fun.


I had the same thing. I was down to zero but stalled out just before it started to go backwards. soooo close....

It is an awesome experience though.

[twr75]

Flying into Melbourne with the notorious low level northerly jet I've had reference groundspeeds in the region of 90 knots!

So Australia Day 2006 would have been doubly fun! :-)

40+C and 20-30kt northerlies...

[AJ]

Yep, not to mention the fact that it doesn't normally cut in until approaching Essendon with the associated moderate turbulence!

[Leftseat86]

Yep, not to mention the fact that it doesn't normally cut in until approaching Essendon with the associated moderate turbulence!

How much time does it add to the flight?

[AJ]

About 2 minutes extra to proceed via the star for 34 as opposed to 27, then about 15 seconds extra for the headwind as it only applies for the last few minutes.