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.


3 Answers 3


On the CFM56-7B engine powering the Boeing 737 NG family, the starter and the handcranking input are driven by the accessory gearbox which role is to link some components to the high-pressure shaft (N2 rotation speed). This gearbox is located outside of the engine core, on the fan case.

Accessory gearbox on the CFM56-7B

enter image description here

Picture: CFM56-7 exploded view. Source.

Corresponding schematic view:

CFM56-7B AGB, horizontal and radial shaft, TGB, IGB and engine shafts

Picture: CFM56-7B AGB, horizontal and radial shaft, TGB, IGB and engine shafts. Taken from this answer.

AGB to core shaft

There is a gear train within the AGB with its input gear used to transmit movement between the AGB and the engine core.

The AGB is connected to the N2 shaft by two bevel gear gearboxes:

  • The transfer gearbox (TGB) fixed just aft of the AGB, on the fan frame case. A horizontal shaft links the AGB input gear and the TGB. A radial shaft is also connected to the TGB.

  • The inlet gearbox (IGB) fixed close to the No 3 bearing, in the fan frame hub. One bevel gear is coaxial to the HPC shaft, the other is coaxial to the radial drive shaft.

The radial drive shaft runs within the 9 o'clock strut of the fan frame.

The described mechanism is bidirectional, the AGB is normally driven by the HPT shaft, but when using the starter or the handcranking mechanism, the pads rotate the AGB input gear using the gear train, and the input gear then drives the intermediate shafts and gearboxes.

Handcranking drive pad and starter

On the picture below,

  • The black handcranking drive pad is visible on top (cover removed). It is used to turn the N2 shaft during borescope inspection (the N1 shaft can be rotated by moving the fan blades manually).

  • The starter is the large assembly with the yellow cover protecting the large pneumatic inlet (air is used to turn the starter).

CFM56-7: Handcranking pad and starter

Picture: CFM56-7: Handcranking pad and starter, source Youtube.

During borescope inspection, either a pneumatic motor or a square drive socket is used:

CFM56-7B handcranking drive pad (cover removed)

Picture: Handcranking drive pad and starter. Source: Youtube.

Alternatively on other CFM56 engines the core can be turned directly from a pad on the TGB.

Accessory pads on the AGB

The accessories have allocated pads on the AGB:

Accessories and their allocated drive pads on the AGB

Picture: Accessories and their allocated drive pads on the AGB. Source CFM56-7B manual.

Also: Actual photos of the gearboxes in this answer.

  • 1
    $\begingroup$ 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, 2017 at 13:46
  • 1
    $\begingroup$ This answer gives a pretty good idea how some engines turn internally, via gears. $\endgroup$
    – timo
    Jun 9, 2017 at 15:31
  • 1
    $\begingroup$ Used in Pakistan International Airlines #8303 update (from 05 min 50 secs. Published 2020-05-23). Channel blancolirio by Juan Browne. It is acknowledged at 07 min 59 secs. Pakistan International Airlines Flight 8303. $\endgroup$ May 24, 2020 at 21:08
  • 1
    $\begingroup$ @PeterMortensen: Thanks for telling, I appreciate. On the CFM56-5B powering the A320 family the AGB is at 6 o'clock and the radial drive shaft is vertical, but otherwise the design is very similar (on this picture a 7B at the foreground, a 5B at the background). $\endgroup$
    – mins
    May 25, 2020 at 11:54

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.

  • 1
    $\begingroup$ Does this mean that on the CF6 the accessory drive has to be exposed to index the shaft? $\endgroup$
    – mongo
    Jun 9, 2017 at 13:38

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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