Honeywell invented the auxiliary power unit (APU) in the early 1950s and remains the market leader for these products. To this end, product improvement continues to be one of the company's top priorities. This was demonstrated in one of the more popular APUs, the 36-150 line used on a variety of high performance business jets, as Honeywell recognized an opportunity to improve the performance and reliability of the compressor seals.
The problem was that APU compressor carbon seals were occasionally leaking into the gas path causing an oil smell in the cabin which caused both inconvenience and flight delays. With some 3,000 APUs in service, it was clear that something needed to be done, yet the combination of temperature, speed and vibration variables conspired to make a final solution elusive.
The issue was brought to the Global Customer Committee (GCC), an independent group of business aviation professionals that works jointly with Honeywell to bring about improvements in aircraft, equipment, operations and services. The GCC escalated the item to their "Top 25" action item list of priorities.
"We were faced with developing a technology that would not only solve the problem itself but solve it in an affordable way for all parties," said program manager Edward Pellegrino. "The team went to work determined to wrap our arms around this once and for all."
The team incorporated modern day seal technology—a hydrodynamic seal featuring a rotor that utilizes its spin speed (60,000 rpm) to create a thin film of air to minimize frictional heating of the seal's carbon element and prevent the temperature that was contributing to the leakage. Additional system modifications were incorporated to address other variables to ensure that the seal capabilities remained robust under all operating conditions.
After extensive development to validate the entire APU operation spectrum with the seal and system components, success has been demonstrated in over 1,000 hours of APU endurance testing and the solution has received FAA approval.
Service bulletins covering the specific 36-150 model variants will be published over the next 18 months. Operators of aircraft equipped with the 36-150 APU can inquire about service bulletin availability through the Honeywell MyAerospace portal. The link can also be used to see what other product improvements Honeywell is working on.