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DME Requirement

Asked by: 5036 views General Aviation

Why is DME required 24000 msl and above? Why 24000 MSL instead of 23000 MSL, 25000 MSL, etc……?

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2 Answers



  1. John D. Collins on Apr 04, 2013

    Positive Control Airspace used to be at FL240 and above. It has since been lowered to FL180 and is now called Class A airspace. The original DME requirement was for FL240 and wasn’t moved to FL180 when the airspace definition changed. I don’t know the precise reason for the requirement, but since most turbo jet fly at or above FL240 and are high performance (high speed), it is more convenient to use distance for intersections than cross radials along the jetways. Also when radar is unavailable due to coverage or outage, having distance available for position reporting allows for simplified along track spacing. It is interesting to note that DME is only required above FL240 if VOR must be used to fly the route. If the aircraft is equipped with a suitable RNAV system, it may use the RNAV in lieu of the DME. Although Turbojet aircraft fly below FL240, they are mostly doing this during the departure and arrival phases of flight and don’t typically cruise below this altitude where slower turboprop and turbocharged aircraft often cruise in the range of the high teens to FL250. Regardless, the requirement for a DME or RNAV is met by the vast majority of all aircraft that fly IFR.

    From 91.205:

    (e) Flight at and above 24,000 feet MSL (FL 240). If VOR navigation equipment is required under paragraph (d)(2) of this section, no person may operate a U.S.-registered civil aircraft within the 50 states and the District of Columbia at or above FL 240 unless that aircraft is equipped with approved DME or a suitable RNAV system.

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  2. Dan Chitty on Apr 04, 2013

    John,

    Thank you for the excellent explanation. I greatly appreciate the feedback.

    Regards,

    Dan

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