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 that have impacted its operations. Here I will discuss the technical challenges of the Chinook, specifically from an engine (Lycoming T55-L-11C) perspective, and its impact on operations. The Lycoming T55-L-11C engine is a turboshaft engine that powers the Chinook, and while a reliable engine, there were three main challenges that needed addressing to keep the aircraft flying.

The first issue was the possibility of turbine disc failure. When the Chinook arrived in Australia, the aircraft could not fly until all the engines went through a Special Technical Instruction (STI) program at a contractor to address issues with “fretting corrosion” associated with the disc/shaft interface. This issue culminated in the crash of A15-011 following a disc failure. The final fix was electro-beam welding of the disc and the shaft by HdH.

1st Stage Power Turbine Failure (A15-011)


1st Stage Power Turbine Failure (A15-011)


A15-011 following engine failure.

The fix, however, was not without its challenges as the techos (Airframe/Avionics and some relief manning for Army and 3AD) spent a lot of overtime working from 0800 2200 most days of the week for several months. The engines being disassembled and the 12SQN Engine Fitters inspecting, reassembling and refitting the engines was an arduous task that took significant time. Large numbers of aircrew were attached out of the squadron for a few months during this technical operation.

The second issue was the 1st stage compressor blades. There was an STI to check blade retention pin engagement, which 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 next issue was Foreign Object Damage (FOD). Due to the nature of the operation, the Chinook would only get very few hours out of an engine before it was damaged. To overcome this issue, it was decided to place a fine mesh screen (fly screen) over the engine inlet. However, there was concern that the critical airflow on the 1st stage might cause a problem. So it was decided to put 25 hours on a pair of engines and send them to the Aircraft Material Research Laboratories at Fisherman’s Bend to check the blades. Hence we went on the “Wyndham Trip” in 1976 for 3 weeks. The plan was to fly to Wyndham with fresh engines and after 25hrs fly to Darwin to change the engines and send them back to AMRL.

On the first day of the Wyndham Trip, 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. After a couple of days, we got to Tennent 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 to drop off fuel for the Army) for a double engine change. After a busy weekend in Darwin, we returned to Wyndham with a “full” aircraft of beer for the Army. 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 it did keep things going. The screens were eventually modified with fine stainless steel mesh.

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 and led to incorrect scheduling of the Variable Inlet Guide Vane. The result was a compressor stall, causing all the compressor blades to break off, 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 properly. Furthermore, we fixed the bellows problem, but that is another story.

Strawb Ellis and I after the last flight of A15-010

Finally, we were able to address all of the issues by introducing the ‘Wide Chord’ blades that were more damage-tolerant and did not require VIGVs. We also implemented a better FOD screen system with the introduction of the CH47-D Chinook and the Honeywell Engine.

When we acquired the ‘C’ model, the T55-L-11C was at its peak of development, producing 4,600 SHP, which was the same as a Hercules engine. The aircraft was capable of lifting the maximum AUW on one engine, which was particularly beneficial in PNG.


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.