Pace vehicle operations
18.104.22.168 Safety considerations
Pace vehicle operations are used extensively at test establishments to test helicopters with relative winds from all directions and a variety of strengths. The operations are relatively straightforward but there are a number of safety considerations that are addressed to ensure that all risks are reduced as far as possible. The operations start with a face-to-face brief between the aircraft crew and the vehicle crew. At this brief the area of operations is defined, including the limits of the area in the form of end markers. As the aircraft may well not be pointing in the direction of its ground track it can be difficult for the pilot to know when he or she has reached the end of the area. When the aircraft reaches an end marker the vehicle crew makes the calls, ‘approaching end marker’ and ‘at end marker’ indicating that the test should be halted. Usually the operations are conducted along a runway as this provides a good surface for the vehicle and an obstacle-free environment for the aircraft. As the aircraft needs to move to different positions around the vehicle when conducting testing it is necessary to reposition the vehicle to different sides of the runway to ensure that the aircraft remains over a clear area. The instructions that will be used to achieve this are briefed. The final call to be stipulated at the brief is the ‘break-off ’ call which the pace vehicle crew will use if for any reason they require the test to be stopped and the aircraft to move away.
Also at the brief the minimum spacing between the vehicle and the aircraft is stipulated. This is usually a minimum distance of two rotor diameters for safety reasons. In practice the actual spacing used is often greater than this to prevent the helicopter downwash from affecting the vehicle anemometer. The ‘golden rule’ for safety in these operations is for the pilot to keep the vehicle in clear sight at all times to prevent a collision. This means that the aircraft will not only have to turn to place the relative airflow at the required azimuth but also will have to move to a new position in relation to the vehicle to ensure that it can be seen clearly. When each test run is complete the aircraft moves well away from the operating area to allow the vehicle to reposition safely. Before starting testing, practice recoveries to the hover from high speed rearwards flight are made; this is particularly important if the aircraft shows a marked nose-down tuck on recovery due to the airflow effect on the horizontal stabilizer.
22.214.171.124 Test technique
Once the pace vehicle has achieved the groundspeed which gives the required test airspeed, the operator calls ‘on condition’. The pilot then positions the aircraft behind the vehicle and lines up with the roof-mounted wind vane; this gives the zero azimuth relative wind and provides the pilot with the heading from which all other relative wind azimuths are calculated. Once the data has been collected the helicopter is turned to achieve the next required wind azimuth, repositioning as necessary to keep the vehicle in sight, see Fig. 5.28. At each test point the pilot ensures that the aircraft is keeping a good formation position with the vehicle before the data is taken. At speeds at or close to the lateral or rearwards limits of the aircraft care is needed not to exceed these limits when repositioning. The technique used is to turn the aircraft into the relative airflow, reposition as necessary, then turn carefully to achieve the required wind azimuth. Making the calculation of the aircraft heading needed to achieve a particular azimuth can be difficult. To ease this process the co-pilot/FTE will often use a swivelling compass rose mounted on a board that has azimuths marked. The aircraft heading when aligned with the wind vane is set against the zero azimuth mark. The aircraft heading needed to generate any wind azimuth can then be read off directly.