Is AD 97-21-09 a recurring Airworthiness Directive?

No — AD 97-21-09 is a one-time directive. Verify it was complied with in the aircraft's records.

Which aircraft does AD 97-21-09 apply to?

AD 97-21-09 applies to Allison — models 250-C47B, 250. Serial/part conditions may narrow it to specific aircraft.

Airworthiness Directive 97-21-09

one-time

Official AD 97-21-09 · FR doc 97-30201 · 1997-11-18 · turboshaft engine

Airworthiness Directives; Allison Engine Company Model 250-C47B Turboshaft Engines

What this AD requires

This amendment supersedes existing airworthiness directive (AD) 96-24-09, applicable to Allison Engine Company Model 250-C47B turboshaft engines, that currently requires replacing the engine main electrical harness assembly with an improved assembly, disabling the overspeed solenoid, inspecting the engine control unit (ECU) internal PW10 voltage to determine electrical noise characteristics, and replacing units not considered serviceable. In addition, the existing AD requires adding a placard to the helicopter instrument panel notifying the pilot that the overspeed protection system is disabled and removes a placard which was required by priority letter AD 96-21- 12; revises the Bell Helicopter Textron, A Division of Textron Canada Ltd. (BHTC) Model 407 Rotorcraft Flight Manual (RFM); and requires maintenance actions to clear the ECU of faults prior to each flight. This amendment continues to require replacing the engine main electrical harness assembly with an improved assembly, but adds the requirements to install a new hydromechanical unit (HMU) and ECU, removing the placard notifying the pilot that the overspeed protection system is disabled, and revises the BHTC Model 407 RFM. This amendment is prompted by the development of overspeed protection system modifications to reactivate the overspeed solenoid in conjunction with raising the power turbine overspeed trip point and revising the overspeed system to default to a minimum fuel flow in the event of its activation. The actions specified by this AD are intended to prevent uncommanded inflight engine shutdowns, which can result in autorotation, forced landing, and possible loss of the helicopter.

Which aircraft AD 97-21-09 applies to

MakesAllison

Models250-C47B, 250

Installed on but not limited to Bell Helicopter Textron, A Division of Textron Canada Ltd. (BHTC) Model 407 helicopters.

Whether AD 97-21-09 actually applies to your aircraft (by serial and installed parts) and whether it’s already complied with comes from that aircraft’s records.

How AD compliance is recorded

A valid compliance record (14 CFR 43.9) states:

Method of compliance — The specific action taken (inspected, replaced, modified) and to what revision.

Date — When the work was done.

Aircraft time — Total time in service / tach at compliance — how recurring next-due is computed.

Signature & certificate — The A&P/IA who returned it to service, with certificate number.

Is AD 97-21-09 complied with on your aircraft?

Tailplate reads a tail number’s logbooks and matches every applicable AD — including this one — to its compliance evidence, with recurring next-due dates computed.

Look up a tail number →Read the full directive (Federal Register)

Applicability is derived from the FAA directive corpus and is advisory, not a legal airworthiness determination. Compliance is established only from an aircraft’s actual records.