Aircraft symmetry and alignment checks are part of the regular maintenance regime so as to localise and identify any structural damage. In this case study an electronic total station and automatic level were used in the lateral and longitudinal levelling of the aircraft.
Symmetry and alignment checks are carried out on aircraft as part of the regular maintenance regime or when the operator/pilot feels it may be necessary after either an accident, excessive airborne turbulence, hard landings or overloading. Verifying the linear and angular symmetry assists in identifying and localising any structural damage.
Fig. 1: Symmetry check – water level method.
The parameters are determined by the aircraft manufacturer and form part of the maintenance and inspection manual. Tolerances are not as severe as may be expected, however, relationships between various parts of the aircraft are critical.
In the past, the aircraft would be jacked up and carefully levelled to a “waterline,” as per manufacturer specification. This would involve jacking the nose and tail (longitudinal levelling), as well as the port and starboard on the wings (lateral levelling). Once the aircraft was level, measurements and levels would be taken from various points to various points and compared to the “waterline” zero and the distances as per specification. The aircraft would have to be in a controlled environment where there is no wind, vibration or movement during the process.
Fig. 2: Control points used in the aircraft alignment.
The important part of this procedure was that the aircraft was elevated off the ground, with the landing gear clear off the ground, and the engines in position. This was done so as to ensure a common base as per the specifications (see Fig. 1).
Modifications would be made by tightening or loosening bolts and anchors, pushing and pulling and inserting wedges and shims with constant re-measurements and checking of the level of the aircraft.
Fig. 3: An electronic total station measuring to reflective targets on the fuselage.
Measurements would be made to these various stations and the comparisons done to the specifications. Should the measurements exceed the tolerances, the aircraft mechanics could opt to strip down the aircraft and replace certain items after which it would again be necessary to re-measure the symmetry points. This procedure could lead to days of adjustment and re-adjustment.To avoid this lengthly procedure, it was decided to establish control points on the aircraft body during the first measuring phase. These control points were then adjusted using the defined waterline as a vertical constraint, the centreline of the aircraft (from nose to tail) as the horizontal constraint, and the aircraft shifted into a north–south alignment (see Fig. 2).
Fig. 4a: Front view of aircraft (Credit: Hawker Beechcraft Corporation).
With each re-measure, these control points would be re-observed as in a resection, and measurements taken to the measuring points to give real-time results. In effect the aircraft was fixed and the total station then shifted into a virtual position relative to the control points. By so doing, the aircraft would be jacked off the ground, but without requiring levelling, adjustments could be made to various fixtures and re-measurements could be made quickly (see Fig. 3).
Fig. 4b: Sideview of aircraft (Credit: Hawker Beechcraft Corporation).
Equipment utilised during these symmetry and aligment checks included a Topcon GPT 7501 reflectorless total station, TopSurve field controller software and Topcon Tools office software. The control points consisted of reflective targets.
Contact Erik van Duffelen, Initio Earth Sciences, Tel 011 795-4007, erik@iesciences.com
