Failure as a Design      Criterion

   Fracture Mechanics

   Failure Analaysis


Wire Rope Failure


Undercarriage Leg Failure


Aircraft Towbar Failure
- Part 1
- Part 2
- Part 3
- Part 4
- Activity 1 - First Hypothesis
- Activity 2 - Fracture Stress


Hail Damage


Insulator Caps


Fractography Resource

Dr David J Grieve ( developed the interactive Java pages for this website.


A Boeing 707 aircraft was being towed from a parking position near the terminal building, to a more remote apron parking. Even in the early 1990's, when this accident happened, 707 aircraft were more occasional visitors to major airports, and the aircraft tractor tow bars were seldom used. On the day in question, the aircraft tow tractor was pulling the aircraft around a 90 turn, on a slight downwards slope, when the tow tractor driver realised that he was being overtaken by the aircraft. The layout of the taxiway in question is shown in Figure 1. Taxiway.JPG (46217 bytes)
Figure 1

Later reconstruction of the sequence of events in this failure indicated that the towbar had failed near the start of the turn, allowing the aircraft to collide with the tow tractor some 20 m later, before finally coming to rest after travelling a distance of some 73 m. These relative positions on the taxiway are indicated in Figure 1. Standard procedure for towing aircraft involves checking the towbar for defects, checking that there is hydraulic pressure available in the aircraft braking system, and having someone travel in the cockpit to apply the brakes in the event of an emergency situation. Whilst the towbar had been inspected visually and such a person was in the aircraft cockpit, the check on the aircraft hydraulic system was not performed and the brakes were ineffective. Thus two main factors contributed to causing this accident; failure of the towbar and failure to ensure that the aircraft taxiway braking system was operative.

The towbar comprised a welded aluminium box section, reinforced internally with angle sections at the corners, with steel coupling sections bolted in the ends. The aircraft end of the towbar is attached to the steel yoke of the aircraft drawbar via two high tensile steel shear bolts, which are designed to prevent high impact forces or drawbar pulls being transmitted to the aircraft undercarriage and causing damage during towing operations.

On examining the towbar, it was found that fracture had occurred in the upper 11 mm diameter shear bolt (marked with the arrow in Figure 2) of the two that attach the steel yoke of the aircraft drawbar to the tow tractor, and that the steel tractor coupling had separated from the aluminium body of the towbar (Figure 3), causing deformation and tearing of the aluminium plates. The consequences of this were that the towbar had 'jack-knifed' around to be parallel to the aircraft as it had run forwards and overtaken the tow tractor.
Aircraft_Coupling.JPG (72245 bytes)
Figure 2 Steel yoke of the aircraft drawbar
Tractor_coupling.JPG (61305 bytes)
Figure 3 Mode of failure of the tractor coupling section
The two coupling ends of the towbar are laid out in their correct relationship to each other in Figure 2 and Figure 3. The evidence therefore suggests that aircraft has overtaken the tractor on the side indicated by the red arrow in the schematic diagram shown in Figure 4. This would cause the damage observed on the towbar at the two couplings, as the towbar jack-knifed round to the left side of the aircraft, viewed in the direction of travel. Schematic.jpg (34071 bytes)
Figure 4

At the time of the failure all the fractured tractor coupling bolts were retrieved, but the shear bolt was not found. All but three of the coupling bolts were subsequently mislaid during metallographic and failure investigation. The shear bolt, however, was found some two months after the accident during routine runway inspection. When discovered, it was in a position further along the runway than that where the tractor coupling bolts had been located.

Proceed to second part of case study.


Failure Analysis  -  Fracture Mechanics  -  Failure As A Design Criterion