Tower fly-by or aneroid method
The tower fly-by method is used to determine the altimeter pressure errors by comparing the cockpit indication with an external measurement of the aircraft’s true pressure altitude. This altitude is found by the summation of the pressure altitude of a fixed datum (base or top of the tower) with the tapeline height of the aircraft above that datum suitably converted to a pressure height. Aircraft height may be measured using a calibrated radar altimeter, a space-positioning system (such as kinetheodolites) or photographically. The method can be improved by connecting a sensitive aneroid to the static system of the aircraft thus identifying errors associated with the cockpit instrument. Care needs to be exercised when handling these aneroids as they are often capable of only tolerating a small altitude range above sea-level and are susceptible to aircraft vibration. Using a radar altimeter is a simple technique that can be used when pre-surveyed facilities are not available. Care must be taken to eliminate the effects of lag and the helicopter should be flown over a smooth level surface to avoid transient errors.
The actual tower fly-by method is normally only used for level flight and hover testing. The aircraft is flown a pre-determined distance from the tower, perhaps down a runway centreline, and is photographed just as it passes the tower. A grid superimposed on the photograph allows determination of the aircraft height. It is common practice for aircraft conducting fly-by testing to have the vertical position of the static ports and any associated transducers or instruments clearly marked as an aid to calculating the true height of the aircraft. Space-positioning equipment may be employed during steady climbs and descents provided a means of synchronizing the data taken in the air and on the ground is available.
Fly-bys are conducted throughout the speed range of the helicopter. It is important that the flight condition is stabilized before data is gathered. Thus an adequate run-in distance must be allowed so that the test height and airspeed can be well stabilized before the instant of measurement. If photography is being used it is important that the aircraft is flown at the correct horizontal distance from the tower otherwise trigonometric errors will be introduced during interpretation of the photographs. As the helicopter approaches a position opposite the reference point the observer in the aircraft records the altimeter reading and radar altimeter indication if appropriate. At the same time a ground observer, at the reference point, notes the static pressure reading from a sensitive aneroid, and photographs the helicopter. A radio link is therefore highly desirable. Both observers keep a record of the runs made so that the test results can be matched at the end of the flight. The aircraft OAT and fuel state should be recorded at the beginning and end of each series of runs. At low IAS singled-engined helicopters may well be operating well inside the avoid curve so time spent in such a condition should be minimized. Tower fly-bys frequently involve use of the primary runway at test establishments and therefore close co-ordination with other activities is required to minimize the risk. As stable weather conditions (low wind and absence of thermal activity or turblence) are a must for accurate testing it is quite often necessary to perform tower fly-bys at dawn or dusk thus easing the problem of deconfliction with other air traffic.
Post-flight data reduction usually uses the convention of relating all the data to the same reference height. Thus the altimeter pressure error is obtained by subtracting the pressure altitude observed in the aircraft, corrected to the reference height, from the pressure altitude of the reference point itself:
*hP = reference pressure altitude — corrected aircraft pressure altitude
Typically the aneroid placed in the aircraft is compared with a similar device placed at the reference height. Obviously each individual aneroid will have a calibration curve that relates the gauge indications to corrected pressure altitudes and these are applied before the pressure error is determined. Errors due to mechanical differences between the aneroids are removed by taking readings from each aneroid when at the same height before and after the test. This so-called ground correction is added to the pressure altitudes recorded in the aircraft:
aircraft pressure altitude = corrected aneroid reading + ground correction
= hpa + (hpr hpa
Although the pilot will endeavour to fly past the tower at the correct height there will often be a small tapeline error, Zc. This error is quantified photographically and if the aircraft is above the camera datum the error will be added. It is important that the tapeline error, or the error plus the height of the tower if the ground aneroid was placed at its base, is converted to a pressure height before the altimeter PE is determined. The correction is achieved by noting the temperature of the air at the reference height and ratioing the tapeline height using the sea-level temperature on the day of the test, assuming an ISA lapse rate, and the ISA sea-level temperature of
288.15 K. Thus:
*hP = hPi — (aircraft pressure altitude
+ pressure height of aircraft above reference point)
= hpr ——— [hpa + (hpr ————- hpa )g + hpc ]
where Tc is the temperature recorded at the reference point on the day of the test and Sc is the temperature difference between the reference point and MSL, assuming a standard lapse rate.