Aero Engine Inspection – 3-Shaft Engine

Most large commercial aero-engines have two separate turbine/compressor “spools” which run at different speeds. The slower low-pressure (LP) spool also carries the fan, the main thrust-producing element of the engine. Because the fan is large, it has a large moment of inertia which means that the LP spool is slow to accelerate or decelerate and makes the engine inflexible to changing throttle demands. The rotational speed of the LP spool is limited by the requirements of the fan.

Some engines use three co-axial spools which allow a large engine to be very flexible. The arrangement avoids the need for moveable compressor stator vanes, a necessary complexity on 2-spool engines. The LP turbine only drives the fan, an Intermediate Turbine (IP) drives what would be the LP compressor on a two-spool turbo-fan, and the HP turbine drives the HP.

The engineering principle involves low, intermediate and high pressure systems, each consisting of a number of compressor and turbine stages, which are mounted on independent shafts which run at their optimum aerodynamic speeds, such as 3,000 rpm, 7,500 rpm and 10,000 rpm respectively. The fan needs to rotate relatively slowly, being limited by the stress and aerodynamic tip speed of the blade.

This arrangement changes the borescope inspection procedure because a three-shaft engine has two compressors (IP and HP,) but three turbines (LP, IP and HP). Furthermore, unlike on a two-shaft engine, turning the fan will not cause the IP compressor to rotate since the fan is driven by the separate decoupled LP turbine.
Some engines are designed for easy borescope or videoprobe Remote visual inspection. However, in engines that have very restricted access severely limits the choice of suitable borescope equipment. Whether the inspection is performed with a videoprobe or a rigid borescope kit, equipment of the utmost portability and compactness is necessary.

Videoprobe : The light-source unit is removed from the system case and taken down the fan duct. All controls are immediately adjacent to the operator and a scope of 2 m length is sufficient. Rival systems on which the processor/light-source is not removable cannot be employed within the fan-duct and must be left outside the duct necessitating a much longer Videoprobe.

Rigid Borescopes : Formerly, the rigid borescope option, a bulky high-intensity light-source was used and left outside the fan-duct. The light brought into the duct with a long (5 m) light guide. If a video camera was employed, a large-screen viewing monitor was also left outside the duct, but was only partly visible to the inspector inside the duct. Extensive cabling ran in and out of the duct. The ELSV-60 Light-source is sufficiently compact and powerful to be taken into the duct alongside the inspector. A standard short (6 ft.) light-guide can be used, greatly enhancing illumination efficiency. If a video camera is required, with a BCAM-1000 hooked to the ELSV-60, cabling is minimized and with a GV-D1000 a large-screen monitor is not required. All equipment is inside the fan-duct.

                  Inspection of a turbine engine