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Examination of a damaged main chassis wheel and other wheel parts from a Boeing 757 aircraft. 1. Introduction 1.1 A request was received for a review to be carried out on a number of documents relating to the in-flight loss of a main chassis wheel from the subject aircraft. The aim of the review was to determine if possible from the available information, whether the wheel had been correctly or incorrectly assembled to the aircraft during maintenance. 1.2 The documents included a police report, the results of which were inconclusive, on an examination carried out on all parts of the subject wheel assembly recovered after the accident and examination of the aircraft immediately after the accident. 1.3 As a result of the document review it was evident, irrespective of differences of opinion regarding the correctness or otherwise of the assembly of the wheel to the aircraft, that the spacer was probably correctly positioned. Statements from the Boeing Aircraft Corporation reported that if the spacer had not been in the correct position, considerable damage to the inner faces of the wheel spokes would have occurred from rotational contact with the adjacent brake pack and the photos submitted with the documents showed that no such damage had occurred. 2. Examination and Experimental Results 2.1 The subject wheel is shown in the as-received condition on Plate 01. 2.2 Visual examination of the wheel revealed that the inner faces of the spokes were not damaged but that the inner end of the bearing housing was missing completely (see Plates 02 and 03 and compare with Plate 18). It was also seen that the outer raceway of the outer bearing had spun in its housing (see Plate 04). 2.3 Examination of the other parts submitted (see Plate 05) provided the following information: a] There were roller skid marks in the outer raceway submitted. From the width of this raceway and skid marks on its inner and outer surfaces it was accepted as being from the outer bearing (see Plate 06). b] The available identity marks on the two inner raceways indicated that the deeper one, which was only slightly damaged and contained the longer rollers, was from the inner bearing and that the other, which was heavily damaged, was from the outer bearing (see Plates 08 to 11 inclusive). c] Examination of the heavily damaged inner raceway from the outer bearing showed that the roller cage fracture had allowed the rollers to cram into part of the track length and seize the bearing. Markings on its bore surface indicated that it had been skidding on the related wheel axle. The inner raceway from the inner bearing did not contain such evidence (see Plates 12 and 13). d] Detachment of the hub cap had resulted from fracture of its attachment lugs by loads in excess of their strength applied rotationally by the rim at the thrust face end of the axle nut (see Plates 14 and 15). 2.4 Assembly experiments on the aircraft using the existing No.8 wheel gave the following results: a] The axle nut had been correctly assembled and locked (see Plate 17). b] Both bearings had been greased (see Plates 18 and 19). c] The wheel hub contained considerable quantities of clear water in the well between the bearings. d] The outer bearing had been severely damaged by corrosion and could not be rotated by hand (see Plates 19 and 20). The inner bearing contained a small quantity of iron oxide in the grease. e] With the spacer positioned between the axle nut and the outer bearing [the position on the axle of the subject aircraft in which it was said to be by some witnesses after the incident], tightening of the axle nut resulted in contact of the inner face of the wheel spokes with the outer face of the brake pack first (see Plates 21, 22 and 23). With the tang washer in position [it was on the axle at the time of the incident] the axle nut would have protruded well beyond the end of the axle and the locking bolts could not have been installed. Even without the tang washer, some four turns of the nut would have been required to position the nut correctly as shown on Plate 17. The torque required to do this would have been greatly in excess of that called for to assemble the wheel correctly onto it's axle and would have virtually destroyed the spokes in the inner part of the wheel and/or severely damaged the brake pack. Rotation of the wheel would have been impossible. 3. Conclusion 3.1 It is considered that the following sequence of events resulted in the in-flight loss of main chassis wheel No.8 from the subject aircraft: a] The bearing housing in the wheel became contaminated with water which collected within the outer bearing in the hub when the chassis was retracted. b] The water in the outer bearing caused extensive corrosion inside the bearing, fracture of the bearing cage and seizure of the bearing by compacting the rollers around part of the roller track. c] Wheel 'wobble' on the axle resulting from raceway displacement within the outer bearing and extensive wear between the bearing outer raceway and the related housing then caused destruction of the part of the bearing housing supporting the inner bearing, loss of the hub cap when the aircraft made a right turn when taxiing out for the incident flight and subsequent detachment of the wheel in-flight due to looseness on the axle. 3.2 It is not considered that the wheel was incorrectly assembled onto its related axle when the wheel was installed.