Groundspeed without GPS

[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.