APU overhaul vs replace: how to decide
When a 737 Classic APU reaches the end of its life, do you overhaul or replace it? Here's how to weigh cost, downtime and remaining life to make the right call.
When the auxiliary power unit on a Boeing 737 Classic, typically the Honeywell/Garrett GTCP85-129, reaches the end of its service life or fails, operators face a familiar decision: overhaul the existing APU or replace it with a serviceable unit. Both can be the right answer. The choice comes down to condition, cost, downtime and the remaining life you end up with.
What the APU does, and why downtime hurts
The APU provides electrical power and bleed air on the ground and, within limits, in flight, running the air conditioning and starting the main engines without a ground cart. An unserviceable APU can mean dispatch restrictions and operational disruption, so the real cost of the decision is not just parts and labour, it is aircraft availability.
Overhaul: when it makes sense
- The core is fundamentally sound and the airframe has significant life left, so investing in that specific unit pays back.
- You want to retain a known-history unit rather than take on an unknown one.
- Shop capacity and turnaround times are acceptable for your operation's downtime tolerance.
Replace / exchange: when it wins
- Downtime is the priority, a serviceable or overhauled exchange unit returns the aircraft to service faster than waiting on a shop visit.
- Your own unit needs extensive, costly work and a sound replacement is available at a better total cost.
- You want predictable cost and a known post-installation status rather than open-ended shop findings.
How to decide
Work the numbers on total cost, the overhaul estimate (including likely findings) against the all-in cost of a serviceable replacement, then weigh downtime and the remaining life each option leaves you with. Confirm condition before committing either way: a borescope-style internal inspection and a full records and LLP review protect you whether you overhaul your own unit or take on another. Danan Aviation helps 737 Classic operators source, purchase and overhaul the GTCP85-129, and models the overhaul-versus-replace decision against your downtime tolerance and budget.
There is no universal answer, only the right answer for this airframe, this budget and how long you can afford to have the aircraft on the ground.