Fly Heli,
I offer the following:.
Our machine uses a Mil Spec Radalt and is available between 0-1650 ft AGL/AHO (Above ht of obstacles) and must comply with the following:
1. Flying S/L altitude retention should be +/- 10 ft for altitudes 0-1650 ft.
2. Now turn the machine at Rate 1 for 180 deg, Steady state altitude retention (b/w 0-1650 ft) should be +/- 10 ft with +/- 30 ft transients allowed.
3. Now turn machine at > 30 deg AOB but < 45 deg AOB, Steady state altitude retention should be +/- 30 ft with +/- 60 ft altitude transients allowed.
This check is done with the AP and trim systems on and SAS on as well. Without an AP or trim system don't know what the requirements would be, however the following is generally accepted.
Rough rule of thumb is 10% of the altitude is the allowable error tolerance eg if you are at 10 ft then a 1 ft error of measuring of the RAD ALT is within tolerance, if you are at 500 ft then an error of 50 ft is within tolerance. The issue is how do you exactly measure 10 ft or 500 ft as the BAR ALT is a waste of space and not accurate enough for the comparison check. Need to hover abeam an object of known (measured) HT and x check RADALT, or use on board radar to determine return (need radar on board to do this).
Have Fun.
LT
