The majority of towbar failures (shear-pins snapping) are typically caused by:
1. Abrupt starts/stops by the Push-back tug operator.
2. Flt crew not releasing brakes when they have advised ground staff that brakes are released and they begin to push while brakes are set.
3. Push-back tug operator not adhereing to the nose-gear steering limit marks painted on the nose-gear door; Push-back operator exceeds nose-gear steering limitation and the bar snaps.
ETA: 4. GSE MX installing incorrect shear bolts when repairing a towbar (as mentioned above, shear bolts have specific ratings), while rare, I have seen GSE maintenance staff accidentally use a lower-rated shear bolt by accident causing premature shearing (again, it's rare, but possible).
Inspection after a towbar incident is routinely required (based upon airline SOP) due to the fact that when a towbar shears, it can easilly make contact with the nose-gear tires and cause damage.
And yes, towbars often vary by aircraft, but many use the same IE: B777, B767, MD11, DC-10 all use the same towbar. The majority, however, are not interchangeable and are labeled for which a/c they will accommodate; they attach by different means, so you cannot connect an MD88 towbar to a B777, two totally different attachment mechanisms.
Last edited by Loadmaster; Apr 30, 2018 at 11:52 pm