View Full Version : identifying DMEs on the B737
SlowAndSilly
9th Jun 2010, 07:00
Hi!
Might sound like a dumb question, but how can you be sure of identifying a DME on the B737?
On the ASP, with NAV selected, how do you know if it is the ILS, VOR or DME you are listening to?
On the NG with its PFD/ND, only VOR/ILS/ADF-ident is decoded into characters as far as I can see from the FCOM.
latetonite
9th Jun 2010, 07:09
Listen to the voice on the paired VOR frequency. It comes on a shared time basis, after the VOR ident.
DME ident is
a) quieter
b) only every 30 secs or so, whereas VOR is more frequent
C) higher pitch
d) rarely listened to
DME ident is
a) quieter
Not sure this is true in all cases. My local DME station audio is much louder.
VOR and ILS frequencies are paired with specific DME frequencies, so it naturally follows that if you have a VOR or ILS frequency selected, you will get the appropriate DME station (and audio).
Are you comparing the NG to other aircraft which may only pair the DME audio with the ILS audio if the Navigation Displays are in APProach Mode?
Does your NG allow you to tune both a VOR frequency and an ILS frequency at the same time on the same panel?
Capt Pit Bull
9th Jun 2010, 08:15
VOR and ILS frequencies are paired with specific DME frequencies, so it naturally follows that if you have a VOR or ILS frequency selected, you will get the appropriate DME station (and audio).
I wouldn't bet on that.
Its not as though there is a mechanical connection, its two separate avionics boxes being signalled by the control head on the flight deck. They can (and do) glitch from time to time. I've had a VOR box hang on a previous freq whilst the DME happily moved on. This would have left us intercepting a VOR radial instead of the ILS centreline, and the VOR was a couple of miles to the side of the airport. Early (non) confirmation of ident made this a non event.
More broadly, people should be in the habit of identifying all aids, DME included. e.g. the near CFIT by a BAe146 following the wrong DME arc.
People get blase about this, because you can go for years and years getting exactly what you think you've dialled up.
pb
In the NG the identification is done automatically and boeing does not see the need to identify navaids via the ASP anymore. It happily identifies DME-only stations as well, not only VORDME or ILSDME ones. Yes, it can happen that it shows a different identification than the DME used, most common when there are two VORDMEs using the same frequency within reception range (TGL and MUN for example).
On the classic you can identify the DME by waiting until you get the morse code on a higher pitch, but you have to plan some extra time for it as it is only every 30 seconds or so.
On the classic you can identify the DME by waiting until you get the morse code on a higher pitch, but you have to plan some extra time for it as it is only every 30 seconds or so. - and the NG!
PS Where did you get 'higher' and '30 seconds' from........................?:)
cdeavionics
9th Jun 2010, 11:24
The ILS audio is 1020Hertz, the DME is 1350Hertz
- and the NG!
Sure you can, but it is neither a boeing recommended SOP anymore nor is it really necessary as the system does it and shows the result on the display system, it even shows if it receives DME only or VOR information additionally.
PS Where did you get 'higher' and '30 seconds' from........................?
ATPL 101 and recently during re-studying the route manual somewhere. As posted above the ident tone for the DME is around 300Hz higher than for the VOR/ILS information.
SlowAndSilly
9th Jun 2010, 15:49
In the NG the identification is done automatically and boeing does not see the need to identify navaids via the ASP anymore. It happily identifies DME-only stations as well, not only VORDME or ILSDME ones. Yes, it can happen that it shows a different identification than the DME used, most common when there are two VORDMEs using the same frequency within reception range (TGL and MUN for example).
The ident you get on the lower part of the ND is VOR/ADF only with VOR/DME readout, according to the FCOM.
The reason I ask, is that on previous aircraft I have flown, the ASP would have separate DME selectors.
GarageYears
9th Jun 2010, 17:15
cdeavionics is correct, ILS idents are 1020Hz tones, while DME are 1350Hz. The difference in pitch is quite different.
For co-located stations, the ILS ident typically is repeated 4 times on an approximately 30 second cycle, followed by a single DME ident, after which the ILS restarts. In other words the idents are interleaved with ILS, ILS, ILS, ILS, DME... and repeat.
- GY
Denti
10th Jun 2010, 00:46
S&S, might be option specific, on ours it identifies DME as well and on DME only stations (quite common here) or if only a DME is identified it will use small characters to show it has only an DME identification.