APU-131 Auxiliary Power Unit — Removal Isolation
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.
How Setyr addresses Aviation MRO →
It refers to equipment that does not exist and illustrates the elements 29 CFR 1910.147(c)(4)(ii) requires a procedure to contain. It is not a document to adopt. Under (c)(4)(i) the employer must develop and document energy control procedures specific to their own machines, and under (c)(6) must inspect them at least annually.
11 points, in sequence
APU master switch and start control on the overhead panel.
Method: Select OFF, install the guarded cover and fit a warning tag at the 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.
APU fuel shutoff valve at the tank feed line.
Method: Close the shutoff valve, lock, and confirm closed at the valve position indicator.
Caution: The APU draws from the same tank as the engines. Closing the engine feed does not close the APU feed.
APU fuel line drain at the low point, downstream of the shutoff.
Method: Drain the line into an approved container and confirm flow has ceased before disconnecting the feed coupling.
Caution: Residual fuel in the feed line spills into the compartment when the coupling is broken.
APU bleed air shutoff valve to the pneumatic manifold.
Method: Close and lock the bleed valve. Allow the duct to cool before contact.
Caution: Bleed ducting operates above 200 degrees C. The burn hazard persists long after shutdown.
APU generator line contactor and its field supply.
Method: Open the generator line contactor, pull and collar the associated circuit breakers.
Caution: A collared breaker is the isolation on an aircraft. Pulling it without the collar invites someone to reset it.
Aircraft battery and APU battery, both isolated at their bus switches.
Method: Select both battery switches OFF and collar. Confirm the DC bus is dead at the test point.
Caution: The APU has its own battery independent of the aircraft battery. Both feed the start contactor.
APU start contactor in the aft equipment bay.
Method: Confirm the start contactor is open with both battery supplies isolated.
Caution: This contactor is what spins the APU. It is the last thing proved dead before the crew reaches into the compartment.
APU fire extinguisher bottle and its squib.
Method: Install the squib safety pin and confirm the bottle is safed before working in the compartment.
Caution: A squib fired with a technician in the compartment discharges extinguishant into a confined space.
Aircraft jacking and shoring, if the aircraft is on jacks for this task.
Method: Confirm jacks are pinned and shoring is in place. Must remain ENGAGED.
Caution: Weight shifts several hundred kilograms aft when the APU comes out.
APU hoist and cradle used to lower the unit from the aft compartment.
Method: Do NOT isolate. Confirm the hoist is rated, rigged and taking the load before the mounts are broken.
Caution: The APU weighs several hundred kilograms above the technician. The hoist takes the weight before the last mount comes out.
Ground power unit at the stand, supplying the aircraft while APU and engines are down.
Method: Do NOT isolate without coordination. Confirm ground power is connected and stable before aircraft systems are needed.
Caution: Removing ground power along with the APU leaves the aircraft with no supply at all, including lighting in the compartment being worked.
What your site must supply
This example is structurally complete and factually generic. The things below are the things only you can know — and they are exactly what makes a procedure yours.
- 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.
Author this properly, once
Setyr holds procedures like this as living documents — bound to real isolation points, with lock registries, group lockout, person-in-charge custody and periodic review built in.