Partial Panel DME Arcs

[aloft]

So I'm doing a little partial panel practice in X-Plane and I decided to shoot an ILS approach with a DME arc for an initial approach segment. At first breath, it sounds pretty hairy but I came up with what I think is a fairly innovative way to maintain the arc in such a situation, wondering if anyone else has thought of this too.

Using the DME groundspeed readout, a groundspeed of zero indicates no movement toward or away from the station; exactly what you want when maintaining a constant-radius arc around the station. By closely controlling that groundspeed readout with brief, shallow banks, I was able to maintain an arc and even make corrections by turning slightly toward the station and monitoring the groundspeed increase to either the outside or inside of the arc. Like GPS, you're dealing only with groundspeed so it's self-correcting for wind.

I haven't come across this technique in any books yet, has anyone heard of this before? Can anyone think of reasons why this isn't a good practice in a partial panel situation? Discuss.

[JEP]

Originally Posted by aloft

So I'm doing a little partial panel practice in X-Plane and I decided to shoot an ILS approach with a DME arc for an initial approach segment. At first breath, it sounds pretty hairy but I came up with what I think is a fairly innovative way to maintain the arc in such a situation, wondering if anyone else has thought of this too.

Using the DME groundspeed readout, a groundspeed of zero indicates no movement toward or away from the station; exactly what you want when maintaining a constant-radius arc around the station. By closely controlling that groundspeed readout with brief, shallow banks, I was able to maintain an arc and even make corrections by turning slightly toward the station and monitoring the groundspeed increase to either the outside or inside of the arc. Like GPS, you're dealing only with groundspeed so it's self-correcting for wind.

I haven't come across this technique in any books yet, has anyone heard of this before? Can anyone think of reasons why this isn't a good practice in a partial panel situation? Discuss.

The only DME I have used has come from the KLN94 so I am not sure I understand the GS of 0 with the DME.

That being said, I would think it would be hard to grasp a ground speed of 0 because you are moving. Maybe not away from the 'target' but you are moving. For me that would get my mind spinning when I am trying to concentrate on the PP approach.

May have to try it though on the sim.

[aloft]

GPS and DME are different paradigms; a GPS will show groundspeed in this situation because it's not deriving its velocity data from a DME antenna. But a GPS will also give you a bearing to the station, which you can use to create a mental RMI of sorts; keeping the bearing to the station at 90 degrees right or left does the same thing as keeping an RMI needle 90 degrees off your nose. The zero groundspeed thing only works for an actual DME receiver, since it only knows velocity based on movement toward or away from the station. When it shows zero, you're maintaining a constant distance from the station, just as you'd want to do on an arc.

[goindownthepipes]

I actually showed this method to my recent CFII student to show how it would work so much better than that "turn ten degrees, twist ten degrees on the CDI plus or minus wind correction." Though I did heavily emphasize that for practical testing purposes that it was probably cheating so teach it the proper way on his flight test. Flight test came and went and as it turns out they did a DME arc. He taught it the correct way but then showed the examiner the trick you have discussed. The examiner actually really liked it especially when the applicant mentioned that the trick only works with traditional DMEs since like you said, a GPS gives you GS all the time where a DME gives it to you based on you speed directly to or from the station.

Will

[fish314]

Maybe you forgot to mention it, but when you said partial panel, exactly what did you still have? As long as you still had the DME readout (in other words, not just the ground speed, but also the actual distance) it sounds like a great technique!

For those that don't have a digital DME with this same setup, it's got the distance on one line and the ground speed on the next line. But it only gives you accurate groundspeed if you are flying right towards or directly away from, the station. If you are arcing, it should display basically zero, because it is basing it's math on how fast DME is decreasing or increasing to determine groundspeed.

[SIUAv8r]

Sounds like a great idea. I think it should be a part of instrument training that you go fly instruments and do DME Arcs and NDB approaches/holds without GPS in an old airplane. These kids today and their hi-tech devices.

[aloft]

Originally Posted by fish314

Maybe you forgot to mention it, but when you said partial panel, exactly what did you still have?

For us bugsmasher drivers, "partial panel" generally refers to loss of the vacuum-driven instruments, usually the attitude and heading indicators.[/quote]

I practiced this a few more times after my initial post, and not only does it enable very pretty, round arcs, it's also useful for your initial turn onto the arc; roll into a standard rate turn from the radial intersecting the arc, and when your groundspeed nears zero, roll wings level.

[fish314]

Ahhh.. That explains it. Cool. (My airplane doesn't have anything vaccum driven! I've just got AC stuff and DC stuff)