Propeller Strike During Towing
During repositioning at a maintenance base, a tug driver misjudges a turn and the right propeller of a twin-engine airplane strikes a steel door frame. The engine was at idle, about 700 RPM, and the propeller stopped within roughly two seconds of contact. One blade has a 12 mm long, 3 mm deep gouge on the leading edge about 150 mm from the tip; the paint stripe at the tip is cracked and there is a light scuff on the spinner dome. The other two blades are unmarked. The propeller is a three-blade, constant-speed, non-feathering unit with aluminium blades. The engine is a direct-drive, turbocharged six-cylinder with about 640 hours since new. The operator points out that the propeller was 'barely ticking over', asks whether the gouge can be filed out and the airplane returned to service, and notes that the annual inspection is not due for another three weeks.
1. What determines whether a sudden-stoppage inspection and engine tear-down are required, and what would you inspect on the propeller itself?
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The controlling question is whether the engine was running at the moment of contact, not how fast. Any blade contact with an object while the engine is turning is a sudden stoppage. The engine manufacturer's sudden-stoppage instructions are the governing document and normally require a tear-down inspection of the rotating and load-bearing assemblies; the propeller manufacturer's blade maintenance manual controls the blade disposition.
Engine scope (typical)
- Propeller flange and crankshaft runout, and flange bolt condition.
- Crankshaft, counterweights, dowel bolts and crankcase fretting or cracking.
- Main and connecting-rod bearings, gear train and accessory drives.
- Ignition timing and magneto condition; engine mount, nose case and controls; firewall and adjacent structure.
Propeller scope
- Damage assessment against the manual's limits: depth, length, location relative to the tip and leading edge, blend-out limits, minimum blade thickness and width, and whether the tip is repairable at all.
- Fluorescent penetrant or other NDT as the manual directs, blade track, hub and blade-bearing inspection, spinner and attaching bolts.
- Balance check or rebalance after any repair.
Low RPM reduces energy but does not eliminate the load path. The inspection scope is set by the manual, not by the operator's estimate of severity.
2. The operator argues that idle RPM means there was not enough energy to damage anything. How would you answer, with reasoning rather than assertion?
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The argument confuses speed with stored energy and ignores how the load is transmitted.
- Rotational energy depends on the square of rotational speed but also on the inertia of the whole assembly. At 700 RPM a blade of about 1.9 m diameter has a tip speed in the order of 140 m/s. That is an enormous tangential velocity at the point of contact.
- The load does not simply stop at the blade. A blade impact applies a bending moment at the blade root, a torque reversal through the hub and a shock load into the propeller flange, crankshaft, counterweights and gear train. These are the components that fail later, not at the moment of impact.
- Damage is often invisible: crankshaft runout within limits on a bent shaft, counterweight fretting, a cracked dowel bolt or a bearing brinell mark will not show on a walk-around.
- The failure mode is catastrophic and in flight: shaft failure, counterweight release or propeller separation. The consequence is out of all proportion to the modest cost of an inspection.
I would also point out that the annual inspection being three weeks away is irrelevant. The aircraft is unairworthy now, and the inspection required by a sudden stoppage is not something the annual will pick up automatically. The decision is technical, not commercial, and it is mine to make.
3. Who can approve the work, and what records must follow the repair or replacement?
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Approval
- Propeller blade repair is treated as a major repair under most national maintenance rules (it appears explicitly in the FAA's Part 43 Appendix A list; equivalent provisions exist in ICAO-based national regulations). It requires approved data - the propeller manufacturer's blade maintenance manual - and an appropriately rated repair station or authorised inspector to approve and return to service, with the required major-repair form where the national rules demand one.
- The engine sudden-stoppage inspection must be performed and certified by an appropriately rated AME/mechanic, with an authorised inspector approving return to service where the rules require it.
- If the blade is beyond limits, the propeller is replaced with an approved assembly; the replacement is not a repair and the records must show the new part and serial number, the approved data and the identity of the propeller.
Records
- Description of the event, engine RPM, duration of rotation after contact and the components involved.
- Inspection findings with measured values: runout, NDT results, blade dimensions against limits.
- Disposition of each component (serviceable, repaired, replaced) with part and serial numbers.
- Approved data reference, signature, authorisation number and date of return to service.
- Notification to the owner in writing, and an occurrence or damage report to the authority if the event meets the reporting threshold.