Setyr worked example — APU-131 Auxiliary Power Unit — Removal Isolation Not approved for use. Version 1 · 2026-09-25
Worked example only — your site must author, review and approve its own
procedure under 29 CFR 1910.147(c)(4). setyr.com/procedures
APU-131 Auxiliary Power Unit — Removal Isolation
Aviation MRO
APU removal from the aft fuselage. Covers fuel feed isolation, bleed air, the APU battery and start contactor, fire bottle safing, and the ground services that must remain available while aircraft power is down.
This is a worked example, not a procedure you can adopt. It describes equipment that does not exist, with illustrative tag numbers.
29 CFR 1910.147(c)(4) requires the employer to develop, document and
utilise energy control procedures specific to their own machines, and (c)(6)
requires the employer to inspect those procedures periodically. Neither duty
can be delegated to a vendor or discharged by this document.
Isolation points
| # | Tag (illustrative) | Type | Disposition | What it does and why | Dwell |
|---|---|---|---|---|---|
| 10 | APU-131-MSTR | Switch | Isolate & lock | APU master switch and start control on the overhead panel. Caution: The APU can be commanded to start from the flight deck by anyone who does not know maintenance is in progress on it. Select OFF, install the guarded cover and fit a warning tag at the panel. | — |
| 20 | APU-131-FUEL | Valve | Isolate & lock | APU fuel shutoff valve at the tank feed line. Caution: The APU draws from the same tank as the engines. Closing the engine feed does not close the APU feed. Close the shutoff valve, lock, and confirm closed at the valve position indicator. | — |
| 30 | APU-131-FUEL-DR | Drain | Isolate & lock | APU fuel line drain at the low point, downstream of the shutoff. Caution: Residual fuel in the feed line spills into the compartment when the coupling is broken. Drain the line into an approved container and confirm flow has ceased before disconnecting the feed coupling. | 300s |
| 40 | APU-131-BLEED | Valve | Isolate & lock | APU bleed air shutoff valve to the pneumatic manifold. Caution: Bleed ducting operates above 200 degrees C. The burn hazard persists long after shutdown. Close and lock the bleed valve. Allow the duct to cool before contact. | 600s |
| 50 | APU-131-GEN | Breaker | Isolate & lock | APU generator line contactor and its field supply. Caution: A collared breaker is the isolation on an aircraft. Pulling it without the collar invites someone to reset it. Open the generator line contactor, pull and collar the associated circuit breakers. | — |
| 60 | APU-131-BATT | Switch | Isolate & lock | Aircraft battery and APU battery, both isolated at their bus switches. Caution: The APU has its own battery independent of the aircraft battery. Both feed the start contactor. Select both battery switches OFF and collar. Confirm the DC bus is dead at the test point. | — |
| 70 | APU-131-START | Breaker | Apply & verify | APU start contactor in the aft equipment bay. Caution: This contactor is what spins the APU. It is the last thing proved dead before the crew reaches into the compartment. Confirm the start contactor is open with both battery supplies isolated. | — |
| 80 | APU-131-FIRE | Block | Apply & verify | APU fire extinguisher bottle and its squib. Caution: A squib fired with a technician in the compartment discharges extinguishant into a confined space. Install the squib safety pin and confirm the bottle is safed before working in the compartment. | — |
| 90 | APU-131-JACK | Block | Apply & verify | Aircraft jacking and shoring, if the aircraft is on jacks for this task. Caution: Weight shifts several hundred kilograms aft when the APU comes out. Confirm jacks are pinned and shoring is in place. Must remain ENGAGED. | — |
| 100 | APU-131-HOIST | Block | KEEP ENERGISED | APU hoist and cradle used to lower the unit from the aft compartment. Caution: The APU weighs several hundred kilograms above the technician. The hoist takes the weight before the last mount comes out. Do NOT isolate. Confirm the hoist is rated, rigged and taking the load before the mounts are broken. | — |
| 110 | APU-131-GPU | Disconnect | KEEP ENERGISED | Ground power unit at the stand, supplying the aircraft while APU and engines are down. Caution: Removing ground power along with the APU leaves the aircraft with no supply at all, including lighting in the compartment being worked. Do NOT isolate without coordination. Confirm ground power is connected and stable before aircraft systems are needed. | — |
What your site must supply
- Your own asset and isolation-point tag numbers. The tags shown are illustrative and refer to equipment that does not exist.
- The actual location of every disconnect, valve and vent on your equipment.
- Measured stored-energy magnitudes and the dwell time required to dissipate them.
- Your verification method for each point, and who is authorised to perform it.
- Any additional energy sources present on your equipment and absent here.
Authorship and validation
This example was authored by Setyr for domain validation. It is not derived from any customer's controlled documents. It illustrates the elements 29 CFR 1910.147(c)(4)(ii) requires a procedure to contain — it does not certify compliance, which is a property of an employer's programme rather than of any document.