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.


On the CFM56-7B engine powering the Boeing 737 NG family, the starter and the handcranking input are part of an accessory gearbox which role is to support components rotating with the high-pressure shaft. This gearbox is located outside of the engine core.

Other engines have a similar design, but the accessory gearbox location may vary, e.g. on the CFM56-5B powering A320 family and 5C powering A340, accessories are at the 6 o'clock position because clearance between nacelle and ground is sufficient.

Accessory gearbox on the CFM56-7B

The accessory gearbox (AGB) is located on the side of the fan containment case.

enter image description here
CFM56-7 exploded view. Source

It is a set of drive pads hosting accessories, including:

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

  • The starter, which is the large assembly at the bottom of the picture.

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

During borescope inspection, a square drive socket attached to a breaker bar is inserted into the handcranking drive pad, or a pneumatic motor is used.

CFM56-7B handcranking drive pad (cover removed)
Handcranking drive pad (cover removed). Source: Youtube

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

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.

Let's remove some components to better see the link from AGB to high-pressure shaft:

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

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.

Pad use

The accessories have allocated pads on the AGB, the ones mentioned have been highlighted in green:

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

Rpm ratio of the gears

Data taken from the engine manual:

  • N2 is 14,460 rpm
  • Radial drive shaft spins at 1.343 N2 (19,418 rpm)
  • Horizontal drive shaft at 1.301 N2 (18,811 rpm)
  • Starter shaft at 1.002 N2 (14,494 rpm)
  • Handcranking shaft at 0.986 N2 (14,260 rpm)

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 '17 at 13:46
  • 1
    $\begingroup$ This answer gives a pretty good idea how some engines turn internally, via gears. $\endgroup$
    – timo
    Jun 9 '17 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 '20 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 '20 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 '17 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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