Most early models of the Learjet 45 did not have an Auxiliary Power Unit or APU onboard. When Bombardier Learjet released the Learjet 45 XR, it included a Learjet 45 APU on the aircraft. All Learjet 45 XR jets have an Auxiliary Power Unit or APU onboard. For Learjet 45s that don’t have an APU, the Learjet 45 APU is available as a retrofit to older Learjet 45s. The unit is mounted in the tail of the aircraft and provides electrical power to the aircraft when on the ground.

Honeywell RE 100-LJ APU – The Bombardier Learjet 45 APU

When Bombardier decided to incorporate an APU on the aircraft, they selected AlliedSignal to develop the APU for the jet. AlliedSignal became apart of global aerospace and industrial firm Honeywell in 1999, and the name of the APU now contains the Honeywell brand.

The Learjet 45 APU is the Honeywell RE 100-LJ APU - Auxilary Power Unit, formerly called the AlliedSignal RE 100-LJ APU. This APU is installed on all original Learjet 45 XR aircraft. It is also available to be installed on Learjet 45s that do not have an APU.

As with all APUs, the Learjet 45 APU is certified under TSO-C77A Category I in the United States. TSO stands for Technical Standard Order, and they are minimum standards that are set by the Federal Aviation Administration for a set of aircraft parts.

Used on the Ground Only

According to the Federal Aviation Administration type certificate for the jet, the APU can only be operated on the ground and not in the air. Also, the APU must be operated at field pressure altitudes that are 10,000 feet or less.


Below are the Honeywell RE 100-LJ APU specifications:

RE 100-LJ Dimensions: 27.2 inches x 17.3 inches x 16.1 inches
Maximum Total Weight: 79 pound (lbs)
Electronic Control Unit is Digital
Basic FAA TSO-C77a, Specification: Category 1
Bleed Airflow: 35 lbs/min @ 36 Pounds Per Square Inch Gauge (PSIG)
Rated Pad Shaft horsepower: 17 SHP
Rotor Speed: 70,200 RPM

Also, note that the Bombardier Learjet 45 APU is also the Bombardier Learjet 45 XR APU and the LJ45 APU.

Learjet 45 APU RE 100-LJ Photo

Above photo of the Learjet 45 APU from Honeywell. Photo Copyright Honeywell. Used under the fair use provision.