DME Speed Accuracy RNAV

The aircraft uses the change of range to a DMEDMEDME —Distance Measuring Equipment —Distance Measuring Equipment over time to calculate its DMEDMEDME —Distance Measuring Equipment —Distance Measuring Equipment speed. This is an entirely separate and different process to the way an ASIASI —Air Speed Indicator calculates speed, which is covered in Instruments. So if the DMEDMEDME —Distance Measuring Equipment —Distance Measuring Equipment range is, say 500 nm and after a minute it has changed to 499nm, that is one nautical mile per minute (or in this case, 60kts).

The problems with DMEDMEDME —Distance Measuring Equipment —Distance Measuring Equipment speed arise when the aircraft's DMEDMEDME —Distance Measuring Equipment —Distance Measuring Equipment distance is not changing at the same rate as its actual speed. This can happen in three situations:

1. If the aircraft is flying tangentially — for example, in a circle around the DMEDMEDME —Distance Measuring Equipment —Distance Measuring Equipment — the range to the DMEDMEDME —Distance Measuring Equipment —Distance Measuring Equipment will not change and so the DMEDMEDME —Distance Measuring Equipment —Distance Measuring Equipment will not pick up any speed at all.
2. If the aircraft is directly overhead, then all of the speed is again tangential and the DMEDMEDME —Distance Measuring Equipment —Distance Measuring Equipment speed will read zero.
3. If the aircraft altitude is very high compared to the plan distance, the aircraft speed will be mainly tangential and so less than its actual speed.

Note that DMEDME —Distance Measuring Equipment speed is always less than or equal to its actual speed.

The DMEDMEDME —Distance Measuring Equipment —Distance Measuring Equipment speed is most accurate when the whole of the aircraft speed is being picked up in the change of distance, when the aircraft is:

• At low level
• Far away from, and
• Flying directly towards or away from the DMEDME —Distance Measuring Equipment.

There are multiple variants of this question on the exam, some including figures - if you remember the three points above, the answer will present itself naturally.