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I'm doing an assignment for my Level 3 Aeronautical Engineering course and stumbled on this question and I can't seem to answer it (we have had no teacher for 4 months and therefore we have to learn by ourselves):

Name and explain the type of coupling between the main engine rotating assembly and the accessory gearbox and therefore how a gas turbine engine is rotated for:

a) engine starting
b) internal inspection (boroscope)

Any help is greatly appreciated.

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    $\begingroup$ I hope you aren't paying good money to a "college" to learn by yourself without a teacher for 4 months... $\endgroup$ – Ron Beyer Jun 8 '17 at 18:47
  • $\begingroup$ No i go for free, and its only our gas turbines teacher $\endgroup$ – Abdu Ayoub Jun 8 '17 at 18:53
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Having done some engine boroscopic inspections in the past, usually a pneumatic drive motor of some type is used to rotate the inner workings of the engine under inspection. On the General Electric CF6-6 and CF6-50 series of turbine engines, there is a drive pad at the 6 o'clock position where the tower shaft that drives the accessory gearbox is located. The cover plate is removed and the pneumatic drive motor (which is usually included in the borescope kit) is hooked up, and the compressor and turbine components can be rotated during the inspection. On smaller turbine engines, usually the inner workings are rotated by hand, using an old-fashioned socket wrench and an extension bar.

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  • $\begingroup$ Does this mean that on the CF6 the accessory drive has to be exposed to index the shaft? $\endgroup$ – mongo Jun 9 '17 at 13:38
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I understand your question is about the mechanism between the main shaft of the engine and the accessory box, mechanism which is also used to start the engine and turn spool during inspection.

This pertains to the HP (IP) spool, as the LP spool is easily turned using the fan which is freely accessible.

I used images from a very good book on engines: The Jet Engine, Rolls Royce


While there are different solutions, the widely used one consists in one or more shafts perpendicular to the high pressure spool (intermediate spool for a Rolls-Royce 3-spool engine) and linked to the latter by a gearbox.

Such driveshaft is known as a radial (or tower) driveshaft.

enter image description here
Radial shaft linked to the intermediate spool in the compressor section

The details of the gearbox between the radial shaft and the shaft may vary:

enter image description here

The other end of the radial driveshaft drives the accessory box using a second gearbox and a driven shaft:

enter image description here

The starter is, as are the accessories, connected to this driven shaft. This allows for the starter to run the spool when starting the engine.

When the engine is not running, the spool can be rotated for inspection by turning another shaft and a reduction gearbox (to facilitate manual rotation). This additional shaft can be accessible from the outside after removing a panel (as it is on the last figure, look for "engine hand-turn access" on the right side).

Actual design may vary, but the principle is common. An overall view of the very used CFM56-7 turbofan:

enter image description here
Radial shaft linked to the spool. Source

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  • $\begingroup$ Many engines do not have externally accessible gearboxes, and boroscopic exams are frequently done with the engine in situ. While your image is from a recognized text, it does not reflect the diversity of configurations, and can be misleading. $\endgroup$ – mongo Jun 9 '17 at 13:46
  • $\begingroup$ This answer gives a pretty good idea how some engines turn internally, via gears. $\endgroup$ – timo Jun 9 '17 at 15:31
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Engine starting: Normally by a starter generator, but some engines are air start.

Borescope inspection: It varies quite a bit. Some have tooling on the shaft which can lock the shaft, and I have seen others without any way of holding the shaft for an in situ inspection. I this case, there are normally two mechanics working...one holding, and the other inspecting. If the engine is removed from the aircraft, then there are clamping points, but I can say that I have seen engines without any, and improvised clamping by the mechanic. I do not know but there may be some manufacturer's guidance on clamping, because obviously one could clamp in the wrong spot.

After my last suspected FOD on a turboprop (PT6), two company mechanics did a borescope inspection. One held / manipulated the prop, while the other threaded the borescope.

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