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DME testing & accuracy standards

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Aircraft Systems, FAA Regulations, General Aviation

Is there any kind of regulation, FAA order, etc. that deals with the accuracy of DME?  Going back to my King IFR course (on VHS!) and the book, they say that when checking the DME during a VOR check (for the purpose of IFR), the acceptable error is ½ mile or 3%, whichever is greater.  AIM 1-1-7 b states, "Reliable signals may be received at distances up to 199 NM at line−of−sight altitude with an accuracy of better than 1/2 mile or 3 percent of the distance, whichever is greater," which I take to mean the overall DME system of transmitter + receiver, but it doesn't say anything about a requirement to test it or what to do in case it falls outside of spec.  In my searching for information I came across AC 90-100A which in Appendix 1 part 4 b it lists what appear to certification standards for DME sensor accuracy which manufacturers must follow, which has changed over the years from ½ mile/3% down to .17 mile/.25%.


So, is there anything official that deals with this matter of checking a DME (particularly for IFR use) and which states what to do if the DME receiver in the airplane is showing a result outside that ½ mile/3% acceptable error from a known distance?  Thanks!

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2 Answers

  1. John D Collins on Jun 30, 2016

    DME distances are only displayed to the nearest NM or 0.1 NM. DME systems that comply with the TSO requirements meet the RTCA DO-189 standard. It has an accuracy requirement of +/- 0.17 NM. There is no airborne test mandated by any regulation in order to use DME for IFR.

    AC 90-100A deals with using RNAV in lieu of conventional navaids. In most cases, the RNAV sensor is based on GPS. However, there is a technology found in high end navigation systems used mostly by the airlines that is based on triangulation of multiple DME’s to determine RNAV position. These systems use a database of DME station locations and frequencies along with multiple simultaneous DME receivers to triangulate the position in terms of latitude and longitude and use this information to provide RNAV point to point navigation. This equipment is normally called DME-DME and either used independently or in conjunction with inertial navigations systems. It is usually done under the covers of an FMS system aboard an airliner or turbojet.

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  2. Skyfox on Jul 07, 2016

    So if I’m above FL240 and navigating by VOR, which requires DME, or flying an instrument approach that requires DME such as the VOR/DME RWY 24 at KJXN, there’s no requirement that I verify the operation and accuracy of my DME? That seems quite strange to me, but ok. For my own peace of mind I’d want to test it once in a while in VFR conditions just to make sure everything’s cool before I fly it into the soup.

    Thanks for the info about the RNAV and DME-DME systems, John. I’ve never flown anything that had RNAV, and I highly doubt I’ll ever get into the front seat of something with DME-DME.

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