business jet engine vs commercial engine maintenance figure and design 2

[gearedfun]

hello
I'm new in the forum and I'm searching for some information related to business jet vs commercial engine comparison. Due to the different usage rate (around 600 hrs/year), can someone tell me wich are the TOW for engines such as PW308, LF507 and CFE738?
thank you.

furthermore, does someone know if the accessory gearbox architecture change? on commercial aircraft dynamic face seals, bearings with 100000 hrs of L10 and high performance stells are used; in the case of business jet does someone know if lip seals are used instead of face seal and if bearings are derated?

thank you

gearedfun

 

[tbuelna]

gearedfun,

I can only respond to your question about EMAD seals and bearings.

Current practice with aircraft rolling element bearings is to rate them for "on condition" life. What this means is that they are kept in service until there is an indication from the HUMS (chip detector, accelerometer, etc) that a bearing failure is occurring. The failure modes of rolling element bearings are very well known and easy to detect early on. They also tend to be very benign and gradual in nature. I've never seen any EMAD bearing designed for an adjusted L10 of 100K hours, that seems a bit excessive. I would expect less than one-third of that number, but I'm not a bearing specialist. A properly designed EMAD would have all components designed to provide a uniform fatigue life and level of reliability based on some life cycle profile such as a given number of "ground-air-ground" cycles.

As for shaft seals, military EMAD/AMAD's sometimes use carbon face seals, but almost never lip seals. Lip seals have very limited surface contact speed capability and tend to produce fairly high friction. If the AMAD needs a long MTBR like a commercial unit would, then the only practical option are non-contacting labyrinth seals that use bleed air. Military hardware usually sees more frequent service and has a much shorter MTBR than commercial units, so the limited life of a carbon face seal is acceptable. The drawback with labyrinth seals is that they weep oil when there is no bleed air provided (during engine shutdown), so the accessory mounted on that shaft's pad must be sealed to the EMAD case and be provided with an oil drainback.

Hope that helps.
Terry

 

[dgapilot]

Each engine manufacturer determines the life limited components for each engine model. They also specify a recommended inspection/overhaul interval. In the US, each commercial operator uses that as the basis to develop their OpSpecs, and based on service history they can adjust the inspection and overhaul intervals, but not life limits.

For biz jets, in the USA, FAR Part 91 specifies that every turbine powered multi engine aircraft must be inspected and maintained in accordance with an approved continious airworthiness program. The engine portion of that program will be based on the manufacturers recommended program and again, based on service history the operator can request adjustement to the inspection and overhaul schedule, but life limits stay where they are. Since most business aircraft don't fly as much, and the individual fleets are smaller, it takes a lot longer to build the operational history to justify time extensions.

Hope this helps.