Originally Posted by
Lead Balloon
Thanks JRB. Regulatory reference? I'm not saying you're wrong. But I can't find anything other than the 75'. (The acceptable 'split' between 2 on board altimeters is a different tolerance, I believe.)
Appendix ‘E’ of FAR 43, which contains the minimum standards that all altimeters must meet. It includes a table of allowable error at altitudes between sea level and 50,000 feet. Most air data computers on jet aircraft are far more precise than the minimum standards in FAR 43. RVSM certified altimetry systems must meet additional accuracy requirements set forth in Appendix ‘G’ of FAR 91 - but that only applies at altitudes between FL290 and FL410.
The Blackhawk air data system would not be RVSM certified as it could not fly high enough to need it, but the CRJ definitely would be. The CRJ has independent dual digital air data computers, and I assume the Blackhawk does as well.
The “75 foot” requirement does not come from a specific FAR, but from the Airman’s Information Manual (AIM), which states that if the current barometric pressure is set on the ground, that the altimeter should read within 75 feet of the known field elevation at the aircraft’s location or the “altimeter accuracy should be suspect”.
In the US, the altimeters of all civil aircraft that fly under IFR must be tested for required accuracy every 24 months. I assume military aircraft have to meet the same requirement.
FAR 43 Appendix ‘E’ is the basic standard for accuracy, but in the case of something like the CRJ, the AMM (Aircraft Maintenance Manual) Chapter 34 will have additional tests to perform which have much more stringent accuracy requirements set forth than FAR 43.