DME override

[punkalouver]

Can anybody tell me what the override position does on a DME unit?

[Erebus]

On the 747-200s I used to fly, the DME would normally only search out to 200 miles, but would search out to 385 miles in OVRD. This extra distance is handy at high altitude on oceanic crossings with few navaids, but could be confusing in busy areas with lots of navaids e.g. western europe.

[411A]

Likewise with the L1011, longer ranges possible in override.

[Graybeard]

My name tells you why I know this.:) Back in the day when there were more airports, fewer planes, and fewer VOR (and paired DME) frequencies, a range limit was needed to keep from locking on to the wrong DME station. Standard was 70 miles for ILS and LVOR-paired frequencies, and 200 nmi for HVOR-paired.
LVOR is/was a VOR for low altitude navigation.

DME is in the 1GHz frequency band, so range is definitely Line of Sight. Line of sight is limited due to earth curvature, of course, and in miles is 1.23 times the square root of the altitude in feet. Hence, line of sight at 10,000 feet is 123 miles, and at 40,000 feet it's 246 miles.

Line of sight is increased if the ground station is on a mountain top, such as Santa Barbara, KSBA, in California. It's at 4,000 feet elevation with a clear shot over the Pacific ocean to the southwest, so you can add about 60 miles to the range based on own altitude. If you get longer range than that, it's due to tunneling effect in certain atmospheric conditions, usually an inversion, and usually transitory.

You can see also my post #800(?) on the Brasil midair for more on DME and transponders.

GB

[punkalouver]

Thanks for the replies. It sounds like perhaps in the normal position, the DME is intentionally given a shorter range to receive signals so that at high altitude there will not be interference from other stations with the same frequency. But if you are in a more remote area, you can go to override to increase the range. Do you normally oprate in override or just the normal position?

[Graybeard]

Thanks for the replies. It sounds like perhaps in the normal position, the DME is intentionally given a shorter range to receive signals so that at high altitude there will not be interference from other stations with the same frequency. But if you are in a more remote area, you can go to override to increase the range. Do you normally oprate in override or just the normal position?
Sorry, I should have mentioned: flight decks designed since about 1978 have the DME overridden in the aircraft wiring, so, yes, full time override mode is now the norm.

The MD-80 Lamm (System) Schematic has the VHF Nav to DME frequency correlation chart. Ask Sparky to see it.

GB

[Alex Whittingham]

As you are talking about override to give ranges in excess of 200NM I imagine the selection would cause the timegates to sweep through the greater range and, probably, the interrogator power output would be boosted. I'll bow to Graybeard's experience on the second point but it seems logical.

Certain Big Airlines used to operate with OVRD continuously selected which always seemed odd to me as there is little use for DME ranges in excess of 200NM and the selection would increase the chance of beacon saturation in busy airspace.

Added as an afterthought: selecting OVRD would also mean a slower initial lock-on, though not by much.

[punkalouver]

That would appear to be the case now anyways if all new aircraft have automatic override.