AD 90-04-01
Applicability
| Type | Manufacturer | Model | Details |
|---|---|---|---|
| aircraft | The Boeing Company | 747-400 Series | Airworthiness Directives; BOEING Model 747-400 Series Airplanes |
Unsafe Condition
Failure of the APU fire detection system due to the DC power main battery switch being in the OFF position during APU operation or cool down cycle.
AI-generated summary from the source AD text. Verify against the official source before acting.
Required Actions
Add a limitation in the AFM requiring the DC power main battery switch to remain ON during APU operation and cool down; modify the APU electrical power circuit per Boeing Service Bulletin 747-26-2135, Rev 1 within 90 days; remove AFM limitation after completing the modification.
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Compliance Time
Within the next ten days after the effective date of this AD for AFM modification, and within the next 90 days after the effective date for the APU electrical power circuit modification.
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Affected Aircraft
Boeing Model 747-400 series airplanes listed in Boeing Service Bulletin 747-26-2135, Rev 1, dated December 21, 1989.
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Source: Official FAA Source ↗
Retrieved: Apr 8, 2026
Rights: U.S. Government Public Domain
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