DME HOLD on B737

[chulmleigh]

A friend has built a B737 simulator at home and we were trying to make an ILS approach that involved holding the DME from an on-airfield VOR. Unable to find any way of displaying this distance on both PFD's with ILS selected on both NAV boxes. Anyone help out please? I fly an Embraer commercially where DME HOLD is easily acquired but unable to find an obvious way round it on the 737 sim. Many thanks.

[westhawk]

I don't know much about building simulators but maybe knowing how DME hold works might provide a useful clue in solving your problem. I'm sure you know how it works on your plane, but I'll summarize the theory of operation.

When a typical radio tuning unit is used to select a VHF NAV frequency, the paired UHF DME frequency is simultaneously and automatically tuned on the DME transmitter/receiver. There is a specific DME frequency paired to each VHF NAV frequency. When DME hold is selected, the VHF NAV receiver may be tuned to a new frequency while the DME transceiver is left "parked" on the previously tuned frequency. When DME hold is deselected, normal paired frequency switching function is restored.

Perhaps a simple "decoupling" switch might be installed in the circuit between the RTU or the VHF NAV receiver (however your sim is wired) and the DME transceiver to accomplish this function in the sim. Having the switch connected so as to supply electricity to a small indicator light will help with reminding the pilot that DME hold is engaged, perhaps preventing sim users from committing an embarrassing navigational error. Modern RTUs installed in current production jets usually have a "DME HOLD" switchlight and a PFD annunciation to prevent just that. In '70s and '80s vintage installations, I've heard the DME hold switch referred to as "the kill ya switch". Though I don't know of any specific instances, I can well imagine...

I hope that helps.

[BOAC]

chulm - I have not seen DME hold on a 737 since the -200. It may be a customer option but not one I have seen. In my time I used the other Nav set on the VOR for DME. Generally ranges on the VOR are not needed after GS intercept and thus you can normally swap to the ILS on that set at that point.

[chulmleigh]

I think you've answered my question BOAC - there isn't the option to hold DME on the aircraft ! How did you do monitored approaches with a FAF say at 4 miles to check glideslope verification altitude/height on the ILS without deselecting the ILS frequency on box 2? As part of our SOP's we are required to call the passing altitude at the FAF (DME range) to check that a false glideslope has not been captured. Did you use another method ? The simulator is a full representation in all aspects of the aircraft except for motion. Thanks.

[BOAC]

As I said, normally the GP 'check' is at intercept and there PNF can be on the VOR DME to make the check, and switch across then to monitor the approach, not forgetting he/she can see the other ILS display - especially the standby - for monitoring.. It is not normally a problem. An inconvenience, yes.:)

I suppose if you really did need a range check AFTER GS capture, a deft switch back/forth/re-ident would do the trick. Not too different to a 'retune for go-round' requirement?

[Denti]

Altitude crosschecks are done either by waypoint (there is one for the OM or 4NM) or via a beacon on the fix page. You have to be sure there is no map-shift, which is no problem since ANP is displayed on the ND and legs page anyway.

We can't swap back and forth on the scond NAV box as we fly every ILS as a dual channel approach until autopilot disconnect. Switchin to another frequency would lead to autopilot disconnect which might not be what you want.

[BOAC]

DEnti - chulm is talking PRNAV approaches here - no 'ANP' in sight/no 'map' in sight -, but GS checks by range against height - no 'waypoints' in sight either! Using FMC generated points for a GS check is NOT what chulm is asking about - that would be very simple indeed. .

Your dual channel approach is easily done, surely? After all, when do YOU need the second AP engaged and why? "Switchin to another frequency would lead to autopilot disconnect which might not be what you want." will only happen if you have the PNF channel engaged! Are all your landings autoland as well?

[Denti]

No, most are not autolands, although every pilot can do one if he wants to. However by SOP every approach is a dual channel approach from arming APP mode (arm APP, select second autopilot on).

If i saw it correctly he was talking about an ILS DME approach, which is not a PRNAV approach but simply a normal precision approach. For PRNAV (RNP 1 elsewhere) i would think you have waypoints as well as ANP. For a normal ILS you do not need to select the approach mode on the ND either, raw data check can be done by the scales in the PFD.

But even if you do not load the FMC at all you will have ANP displayed on the ND, wont help much without any waypoints though, i give you that :) Anyway, it used to be common and still is for many airports to have a co-located OM/NDB which makes checking the GS very easy, simply watch the needle swing.

[Cough]

I have flown a -500 or two that had DME hold - Very useful in Germany, though you couldn't get an ident from said DME once in the hold function.

In the others, as BOAC says, check DME joining the glide at the FAF, select ILS freq, ident, and go dual channel. If you need the 4dme check, ask ATC to report to you 4dme... If your SOP's say dual channel from selecting approach mode, you have just gotta decide whats more important. SOPS cover most scenarios, but only cover as much as the person writing them can conceive.

[Centaurus]

as we fly every ILS as a dual channel approach until autopilot disconnect.

Interesting concept. I thought dual channel was only for autolands where an auto go-around might be needed from ground level. Do you also have a policy of dual channel ILS approaches even on a bright and sunny day? Seems a bit superfluous.

[BOAC]

Cent - I fear you are well out of touch!

[Denti]

@Cent, yes, as long as any autopilot is used it is, if no autopilot is used it isn't obviously. Was introduced around 20 years ago on request of a Big Airline that owned us back then. Supposedly it reduces the risk of one faulty autopilot and was introduced after some aileron hardovers on approach caused by autopilot malfunctions.

Since all approaches are flown using the approach mode it is one distinction between precision (ILS, GLS) and non precision (the rest) approaches.