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.

  • 1
    $\begingroup$ 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

Let's take the CFM56-7B engine powering the Boeing 737 NG as an example.

Accessory gearbox

The accessory gearbox (AGB) is located on the side of the fan containment case. It is a set of drive pads hosting accessories, including:

  • The starter.

  • The handcranking mechanism used to turn the N2 shaft during borescope inspection (the N1 shaft can be rotated by moving the fan blades manually).

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. 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.

At this stage I guess some drawing is necessary :-)

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

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

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

Regarding the 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)

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

You can also look at this other answer for actual photos of the gearboxes.

Details of the IGB near No 3 bearing:

CFM56-7B IGB and No 3 bearing
CFM56-7B IGB and No 3 bearing

  • $\begingroup$ 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

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.

  • $\begingroup$ 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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