Glideslope callout on take off

[Christopher Low 5,353]

I was only around 100 feet off the ground, so the landing gear was still down.

[downscc 1,327]

16 hours ago, Qavion2 said:

DME transmissions contain an electronic offset to compensate for different ground antenna locations (with respect to the runway threshold). i.e. the transmitter may alter the response time delay to compensate for the antenna location. Depending on the relative position of the aircraft in relation to the antenna, you may get a substantial distance error.

 

Wow.... that is not something you will see in the US or on any naval TACAN.  I guess yanks are not as compulsive about DME reading zero at the threshold, in fact I don't think there is a single ILS in the US that does that.  How very interesting.

The transmitting antenna is simply an omnidirectional "whip" antenna on the ground, in the US it is usually collocated with the LOC transmitter (not LOC antenna) or GS transmitter. Now my curiosity has me looking at EGCC again:  The CATI appears to have the DME zeroed out at the runway threshold and the CATII/III's seem to have DME zero at touchdown.  I'll be darned.

[Christopher Low 5,353]

I have another "pattern" flight lined up for EGCC Manchester this weekend, but this time I will be flying the 777-200F.

[Qavion2 39]

10 hours ago, downscc said:

Wow.... that is not something you will see in the US or on any naval TACAN. 

Strange. That was my understanding of how it worked. The official nav databases have an ILS-DME offset value and they have been incorporated in at least one high end desktop sim.

If anyone is going to notice this, it's light aircraft pilots. Do you normally get 0.0nm at the glideslope transmitter? (allowing for the distance from the runway to the ground antenna).

[Christopher Low 5,353]

I can confirm that the "glideslope callout on takeoff" issue also occurs with the 777 on my PC.

[downscc 1,327]

12 hours ago, Qavion2 said:

Strange. That was my understanding of how it worked. The official nav databases have an ILS-DME offset value and they have been incorporated in at least one high end desktop sim.

If anyone is going to notice this, it's light aircraft pilots. Do you normally get 0.0nm at the glideslope transmitter? (allowing for the distance from the runway to the ground antenna).

Only if the DME is collocated with the GS antenna. Take a look at just about any US airport, say for example KIAH Houston.  Landing any runway East (8L, 8R or 9) the DME is about 0.2 at runway threshold but landing any runway West (26L, 26R or 27) the DME at landing threshold is 1.7-1.8.  This illustrates that the DMEs are collocated with the Eastbound GS transmitter/antennas.  There is no such thing as an artificial adjustment to the DME to set the DME value to zero at threshold or touchdown.  Not in the US.  I cannot speak to other regions.

[JRBarrett 3,070]

29 minutes ago, downscc said:

Only if the DME is collocated with the GS antenna. Take a look at just about any US airport, say for example KIAH Houston.  Landing any runway East (8L, 8R or 9) the DME is about 0.2 at runway threshold but landing any runway West (26L, 26R or 27) the DME at landing threshold is 1.7-1.8.  This illustrates that the DMEs are collocated with the Eastbound GS transmitter/antennas.  There is no such thing as an artificial adjustment to the DME to set the DME value to zero at threshold or touchdown.  Not in the US.  I cannot speak to other regions.

Terminal DME (TDME) does have an offset to produce a zero reading at the touchdown point. I don’t know if they are used in the US, but TDME equipment is very common at European airports.

[downscc 1,327]

2 minutes ago, JRBarrett said:

Terminal DME (TDME) does have an offset to produce a zero reading at the touchdown point. I don’t know if they are used in the US, but TDME equipment is very common at European airports.

Thanks Jim... you put a name to it and sure enough TDME appears in a Wikipedia article on DME.  Correct, I've never seen this at a US location, we are quite used to the DME value being nonzero at threshold or touchdown and I am a little amused that this is even a thing.  Sounds like something dreamed up by a bureaucrat rather than a pilot.

[JRBarrett 3,070]

Just now, downscc said:

Thanks Jim... you put a name to it and sure enough TDME appears in a Wikipedia article on DME.  Correct, I've never seen this at a US location, we are quite used to the DME value being nonzero at threshold or touchdown and I am a little amused that this is even a thing.  Sounds like something dreamed up by a bureaucrat rather than a pilot.

When I googled “TDME”, one hit was on an FAA document setting out qualifications and equipment standards for contractors engaged in calibration of “government owned TDME facilities”.

Based on that, perhaps there are TDME systems at some US airports.

[downscc 1,327]

5 hours ago, JRBarrett said:

When I googled “TDME”, one hit was on an FAA document setting out qualifications and equipment standards for contractors engaged in calibration of “government owned TDME facilities”.

Based on that, perhaps there are TDME systems at some US airports.

Haha.. probably. I have not seen a DME 0 at threshold yet but I've only looked at maybe 80 of the thousands of airports.  Then there is the possibility that the procedure is used to trim the DME value such that if the approach chart says 1.8 DME at threshold then when flight check aircraft flies the approach the DME is really 1.800.  Who knows, I've seen worse ways of spending money by the government.

Edited July 27, 2018 by downscc

[berts 97]

On 7/27/2018 at 10:42 PM, downscc said:

Haha.. probably. I have not seen a DME 0 at threshold yet but I've only looked at maybe 80 of the thousands of airports.

Dan, 

Who cares what the DME reads at the threshold (except for Navy carrier pilots)?  I'm usually too busy looking down the runway when crossing the threshold and hoping I can stop the beast in time before it reaches the other end! 😵

[downscc 1,327]

19 minutes ago, berts said:

Dan, 

Who cares what the DME reads at the threshold (except for Navy carrier pilots)?  I'm usually too busy looking down the runway when crossing the threshold and hoping I can stop the beast in time before it reaches the other end! 😵

That's my point.  This idea has to be cooked up by a non-pilot... and those Navy jocks are watching the meat ball.

[skelsey 765]

11 hours ago, berts said:

Who cares what the DME reads at the threshold

Well - surely it is useful to know what the DME reading at the threshold is going to be in order to make an accurate assessment of one's height vs range to check the glide path and for an accurate indication of distance from touchdown? 

Of course, the DME could read anything at all provided it is a known figure... but 0 would seem to me to be the most logical choice resulting in the least mental gymnastics :).

Edited August 10, 2018 by skelsey

[berts 97]

13 hours ago, skelsey said:

Well - surely it is useful to know what the DME reading at the threshold is going to be in order to make an accurate assessment of one's height vs range to check the glide path and for an accurate indication of distance from touchdown? 

Of course, the DME could read anything at all provided it is a known figure... but 0 would seem to me to be the most logical choice resulting in the least mental gymnastics :).

Simon, I was joking, honest.  At least Dan cottoned on to the fact pilots do care - meat balls and all!  

Mind you, nothing can beat an experienced Mark 1 eyeball and, if you are fortunate to have one, a reliable Radio Altimeter.  Just think back to the days of Apollo 11 and what Neil Armstrong did when his descent computer got overloaded and was no use to him.  He simply took manual control and calmly landed the lunar module without any fuss - and more importantly without a DME!