AD 79-21-01 R1
Applicability
| Type | Manufacturer | Model | Details |
|---|---|---|---|
| engine | Rolls-Royce Corporation | 250-B17 | DETROIT DIESEL ALLISON Model 250-B17, 250-B17B, 250-B17C, 250-C20, 250-C20B, and 250-C20C (MIL. T63-A-720) Engines |
| engine | Rolls-Royce Corporation | 250-B17B | DETROIT DIESEL ALLISON Model 250-B17, 250-B17B, 250-B17C, 250-C20, 250-C20B, and 250-C20C (MIL. T63-A-720) Engines |
| engine | Rolls-Royce Corporation | 250-B17C | DETROIT DIESEL ALLISON Model 250-B17, 250-B17B, 250-B17C, 250-C20, 250-C20B, and 250-C20C (MIL. T63-A-720) Engines |
| engine | Rolls-Royce Corporation | 250-C20 | DETROIT DIESEL ALLISON Model 250-B17, 250-B17B, 250-B17C, 250-C20, 250-C20B, and 250-C20C (MIL. T63-A-720) Engines |
| engine | Rolls-Royce Corporation | 250-C20B | DETROIT DIESEL ALLISON Model 250-B17, 250-B17B, 250-B17C, 250-C20, 250-C20B, and 250-C20C (MIL. T63-A-720) Engines |
| engine | Rolls-Royce Corporation | 250-C20C (T63-A-720) | DETROIT DIESEL ALLISON Model 250-B17, 250-B17B, 250-B17C, 250-C20, 250-C20B, and 250-C20C (MIL. T63-A-720) Engines |
Unsafe Condition
Third stage turbine wheel blade separation due to excessive temperature exposure beyond established limits during hot starts.
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Required Actions
Remove the turbine wheels from service if temperature-time limits are exceeded (810-927°C for over 10 seconds or above 927°C for any duration). Install FAA-approved turbine wheels. Do not rework and reinstall turbine wheels after a hot start.
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Compliance Time
Prior to further flight, except for flights under FAR 21.197 to a base where removal can be performed.
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Affected Aircraft
DETROIT DIESEL ALLISON Model 250-B17, 250-B17B, 250-B17C, 250-C20, 250-C20B, and 250-C20C (MIL. T63-A-720) engines with third stage turbine wheel Part Numbers 6898551, 6898567, 6898733, 6898743, 6898753, 6898763, 6899406, 6899415, 6899416, 6899417, 6899418, 6899419, 6896863, 6898823, or 6899364.
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Source: Official FAA Source ↗
Retrieved: Apr 8, 2026
Rights: U.S. Government Public Domain
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