Hi,
Quick question about the DME. Lets say I am on an IFR flight to a certain airport and it just so happens that my DME fails. In this case, I will not have groundspeed indication since the DME sends signal back and fourth between aircraft and station. So, can I use this formula. To get the time to the station, lets say it takes 10 minutes to move between two radials, so take (10 *60)/10 = 60 minutes to the station. To find the distance to the station, lets say indicated airspeed is 100 KTAS:
(10 * 100)/10 = 100 NM
What if the DME fails while doing arc for an approach, can I just keep the ADF needle at the wingtip till I intercept final track?
Am I on the right track here? If not, please guide me to the right direction. Also, how would I find the ground speed?
Cheers

Those formulas you are using are correct:

Time to Station (in minutes) = Time in Seconds / Degrees of Bearing Change

Distance to Station = (TAS x Time to Station in Minutes) / Degrees of Bearing Change

To calculate the ground speed, I guess you could calculate the distance between the two limits of the bearing change at that distance out from the station and then calculate the ground speed based on the time it took you to traverse that distance - assuming you were flying a constant heading of course. If there's a quick rule of thumb for that, I'd like to know about it.

If you are flying a DME arc as part of an instrument approach, then the DME will be a required instrument and marked as such on the plate. Therefore if the DME failed it would be fair to say you no longer have the required equipment and could therefore not continue the approach.

Some approaches which require a DME may have a note stating that GNSS may be used in lieu of DME so in that case, you could continue as long as you had GPS information available.

Cheers,

Rich