Challenger CL-605 APU
 

Hello All,

I read that the 605's APU could be started up to 20,000ft and utilized up to 20,000ft.
Has anybody replaced or modified the existing APU in order to give the aircraft the capability to use it at a higher altitude (say 30 or 35,000ft) ?
Is it reasonably conceivable ?
Thanks a lot for your valuable inputs,

I would say no because it would make zero financial sense. The cost to design and certify such a modification would be much more than just buying a different aircraft that already has that capability.

However, I don't see the APU limitation as very restricting. What is the reason for wanting to run the APU at higher altitudes?

Let's say if you wanted to use the aircraft not for regular corporate use but for technical/operational missions use.

Buy a G280. The APU can be used up to FL400, started at FL350 and will run the pack up to FL200.

I still think your better off getting the right airframe. Why modify a perfectly good 605 which will most certainly decrease it's value? Buy an early GLEX which can start the APU at 37,000 feet and operate up to 45,000 feet. Maybe a G-IV might be an option also, I'm not as familiar with Gulfstreams.

The reason the 605 can't run the APU at altitude is due to the inlet design. There is no door, your going to have to engineer a new intake and system to control it, and integrate all that into the APU FADEC. You'll see that aircraft that can run the APU at altitude have a APU intake door that can vary the opening angle. It's not a small project.

Even with that, I don't see the reason to need an APU. All your going to get from an APU at high altitudes is electrics, there are probably better routes to getting more electrical power than running the APU.

You can see many GLEXs modified for special missions, I can't think of any 605s. The 605 isn't that capable of an aircraft, it serves it's purpose fine but there isn't much extra in any of the systems for "special missions". Probably the reason you don't see any modified ones.

Well, Boeing is using the 605 for a mini-P-8 maritime patrol version, admittedly a low altitude mission.

I thought it was Field Aviation that were doing the Challenger ASW mod...anyways, I haven't heard that any aircraft are actually in service. The Dash 8 is a better platform for the low altitude roles.

Field's doing the work, but Boeing did the design and sales effort. Actually, the GE engines were designed for low altitude work on the A-10 and S-3.

If the need is for more electric power, perhaps this could be done with an appropriately sized fixed RAT. Of course, there would be some performance penalty, maybe too much, but there would be no altitude restriction.
On resale, remove the fixed RAT and you are back to basic airframe.
Getting approval should be much easier too.