In the formation method, the test aircraft is flown in concert with another (pace) aircraft whose pressure errors are known. This method has several advantages over others:
• Both level flight altimeter and ASI pressure errors can be determined simultaneously.
• The method can be used for climbs and descents.
• Since the PE calibration is made by comparing the altimeter and ASI readings of the two aircraft in the same air mass there is no need to calculate TAS.
The pace aircraft is normally equipped with calibrated pitot and static systems fed from boom mounted sensors to eliminate the effect of main rotor downwash. The pitot head is usually swivel-mounted to reduce the effects of a and p.
The distance between the test aircraft and the pace aircraft is carefully chosen so that it is sufficiently close to aid holding a steady station yet far enough away to ensure no mutual interference. A spacing equal to two rotor diameters of the larger helicopter is often used. Once again the required test conditions are maintained for about one minute while the observers in both aircraft record IAS and height every 15 seconds. Clearly IAS comparisons can only be made if the pace aircraft can cover the speed range of the test helicopter. However, this is not necessary for static errors as the test helicopter can be flown past the pace aircraft (or vice-versa) and altimeter readings compared. As with any formation flying, a comprehensive briefing, which deals with all aspects of normal and emergency operations, communications and safety procedures, is essential. There is often a temptation to fly too close together, which must be resisted if good data is to be gathered with minimum risk to either aircraft. Since the pilot will spend the majority of the time ‘eyes out’, the co-pilot or FTE usually assumes the responsibility for monitoring aircraft systems as well as advising the pilot of slip-ball position. It is important that all changes in flight path are made gently with due consideration to any performance differences between test and pace aircraft otherwise time will be wasted unnecessarily in re-establishing a tight formation.
Reduction of data gathered using the formation method is perhaps the simplest of any method currently used to determine pressure errors. After adjustment of all readings from both the pace and the test aircraft, to take account of instrument errors, the pressure errors are found simply by subtraction. If the airspeed and altitude from the pace aircraft come from test gauges fitted to a boom mounted system operating in the freestream there is usually no need to be concerned with the effect of the pressure field surrounding the aircraft. So:
*V = (ASIR + instrument error)pace — (ASIR + instrument error)test
*hP = (AltR + instrument error)pace — (AltR + instrument error)test