By Bob Medwell

[Main image: The Crew of A15-010 at Wyndham for engine trials and Exercise support]

The Boeing CH-47 Chinook, a tandem rotor helicopter, has served as a versatile transport platform for various military forces around the world. While it is a workhorse of a helicopter, it has faced several technical challenges throughout its service life, non-more so than in the early days of RAAF Chinook operations. Here we specifically discuss the technical challenges of the Chinook’s, Lycoming T55-L-11C engines.

When the RAAF acquired the CH47c Chinook (or ‘Super C’ model), the T55-L-11C was at it’s peak of development, producing 4,600 SHP (at Contingency Power), which was the same as a Hercules engine. With this power, the aircraft could lift maximum AUW on one engine; this was particularly beneficial in PNG.

LycomingT55-L11-C cut-away


Turbine Disc Failure

Prior to Chinooks arrival in Australian, there had been a fatal accident overseas – this accident was believed to be as a result of T55-L11-C turbine failure – specifically attachment the power turbine to the output shaft. As a result of this issue, the first task was to remove all engines and send them to the contractor for rework of the disc matting surfaces; to prevent fretting corrosion).

However, the rework of the shaft/disc interface was not a long terms fix, and in June 1975 A15-011 suffered a turbine disc failure on climb out. From the accident investigation it was proposed to weld the shaft to the disc at the HdH. This fix again resulted in all the engines being removed and sent to the contractor for modification. As a result of the aircraft being grounding a most of the aircrew were attached out of the squadron for several months.

1st Stage Power Turbine Failure (A15-011)


1st Stage Power Turbine Failure (A15-011)


A15-011 following engine failure.

1st stage Compressor Blades.

The second issue was the 1st stage compressor blades. During the period of addressing the turbine issue, there was an STI to check blade retention pin engagement. This required the compressor section to be unbolted (split), and with a special tool check the pin engagement through the little hole in the root of the blade. This process would take about one week per engine.

The third and most frustrating issue was Foreign Object Damage (FOD) to the first stage compressor blades. As a result of ineffective FOD screen system, and the nature of Chinook operation, we would only get very few hours out of an engine before it was damaged. To address this issue, it was decided to trial a redesign screen by attaching a fine mesh overlay (fly screen) on the existing screen. However, there was concern that the critical airflow on to the 1st stage might cause blade integrity issues. To evaluate impact of the fine screen mess, it was decided to fly 25 hours on a pair of engines.

1st Stage Compressor Blade – FOD at the centre of the leading edge

To fly the 25 hours on a pair of engines in an operational environment, it was decided to do this in conjunction with supporting the Army on an exercise out of Wyndham WA. In August 1976 we departed Amberley with a ‘large support crew’ for 3 weeks. The plan was to fly to Wyndham with fresh engines, fit the screen overlays at Wyndham and after 25hrs fly to Darwin to change the engines and send them back to AMRL.

On the first day, we had three chip lights (indicating metal in the oil system) on No.2 engine, and then another on day 2 out of Mt. Isa. We decided to change the Engine Nose Box Transmission as there was no metal on the engine chip detector. After a couple of days, we got to Tennant Creek and FOD’d the No.2 Engine, resulting in an engine change. We finally got to Wyndham after about a week and left for Darwin (via Port Keats Aboriginal Mission) for a double engine change. After a busy weekend in Darwin, we returned to Wyndham. At the conclusion of the exercise, we returned to Amberley via Mt. Isa, where we encountered a large hydraulic leak, resulting in a change of the Pivoting Actuator.

The fine mesh screen approach worked, although the fiberglass fly screen mesh failed regularly but it did keep things going. The existing screens were eventually modified with a fine stainless steel mesh overlay.

Variable Inlet Guide Vane

The third issue that we encountered was the failure of the P2/T2 bellows in the Fuel Control Unit (FCU). This failure was caused by corrosion which led to incorrect scheduling of the Variable Inlet Guide Vane (VIGV). The result was a compressor stall, causing all the compressor blades to break off (FOD screen full of blades), ultimately leading to engine failure. Eventually, we were able to determine the root cause of the corrosion and developed an indication system to ensure that the VIGVs were functioning correctly. Furthermore, we fixed the bellows problem, but that is another story.

Bob Medwell (AC) and Doug (Strawb) Ellis (CPL) after the last flight of A15-010, our first engine installation in 1974


In previous videos it has been mentioned that the RAAF Chinooks where extremely maintenance intensive; it was thought that the Chinook cost the same per hour to operate as a Hercules. However, the Squadron maintenance personnel worked to support each other, working cross-trade and supported by 3AD and Army maintenance teams. Non engine tradespeople would disassemble the engine Quick Engine Change kit, and engines trades people would carry out the repair and reassemble the engine. Furthermore, it was not uncommon to move serviceable engines between aircraft (‘musical engines’) to keep aircrafts going. In the case of engine related issues, it was common to work 14 hour days, several days a week for months on end.

Finally, the T55-L-11C was replaced by the Honeywell T55-L-712 in the 1980s. This engine had ‘Wide Chord’ 1st Stable Compressor Blades which were FOD tolerant and there was no requirement for VIGV system.

Although there were plans to upgrade 12SQN’s C models to D like, history came into play with the transition to Army, and the purchase of CH-47 D Chinook (some were original 12SQN C models).


Bob Medwell was posted to 12SQN in March ’74 from 9SQN after 4 months (ex RAAFSTT). As an AC Engine Fitter he grew up technically in the SQN, and was posted out in Jan ’81 for System Technician training (RMIT). This was followed by F-111s from Dec ’82 to Jan ’87. He was commissioned in 1988 – HQLC, RAAFSTT, Canberra and then as Technical Trade Sponsor at HQLC/DGTA Laverton.