Adventures in accident investigation: Innovation for answers
14 July 2017
Posted by Barry Holt
Getting to the why and how of an accident can be an adventure. Digging deeper to discover how to make a complex aircraft system safer is where the game can get really challenging. It can also be a test of a team's ingenuity and creativity. Let me give you an example from a recent investigation report released by the Transportation Safety Board of Canada (TSB).
On 6 November 2014, a Bombardier Q400, operating as Jazz Aviation Flight 8481, left Calgary International Airport for Grande Prairie, Alberta, with 71 passengers and four crewmembers on board. During take-off, the number 3 tire failed (that is the inboard or left tire on the right side). The Calgary control tower notified the flight crew that debris from the number 3 was found on the runway. The flight crew assessed the situation, consulted their maintenance and operations people and it was decided that the flight would divert to the Edmonton International Airport (CYEG) to land.
Throughout the approach phase of landing, the landing gear control panel indicated that the landing gear was down and locked. The aircraft touched down very lightly on Runway 02 at CYEG. Almost immediately after initial touchdown, a pronounced vibration shook the aircraft; while the nose wheel of the aircraft was still airborne, the right main landing gear (MLG) collapsed. The right-side propeller blades struck the runway, and all were sheared on contact. One large section of a propeller blade penetrated the aircraft cabin next to a passenger in row 7. At the same time as the propeller contacted the ground, the nose landing gear came down hard and its tires ruptured. Thankfully, there were only a few minor injuries.
We started out knowing that a tire had failed. Blowouts are not common, but they are not unheard of either. But when a MLG fails, it is important to know how and why.
The TSB investigation team worked in collaboration with engineers at UTC Aerospace Systems/Bombardier to develop an investigation plan that encompassed all parts of the landing gear system and its control. Since components of aircraft are often outsourced beyond the manufacturer of the aircraft itself, this required bringing together individuals with backgrounds in electrical, avionics, design and manufacture, tire design and failure modes, maintenance, and operations. The expertise came from a number of different organizations, each with unique agendas, ideas and priorities. At the outset there was some resistance to the idea that the landing gear wasn't functioning as designed. Clearly it had not. Once all of the players came together, the team buckled down to the goal of discovering how the landing gear failed.
As soon as the aircraft was recovered to a hangar for inspection and repairs, the aircraft landing gear and all involved systems were inspected and their functions were checked while the aircraft was on jacks. Both shock struts were found to be correctly serviced, and all grease points were adequately lubricated. No components had visible damage, except for the failed retraction actuator that retracts and extends the landing gear. It had failed under a compression load and, for testing purposes, was replaced with a serviceable unit. The nose landing gear was cleaned and repaired to the extent necessary, and new tires were put on the aircraft, also to allow for the testing. Then the landing gear was operated to the retracted and extended positions a number of times. Both the main and the nose landing gear systems performed correctly and within specifications. So what failed? This was a challenging question.
Vibration testing models of all the landing gear components were developed and agreed to by the investigation team. This process and the subsequent vibration testing had not been required when the landing gear was certified so the team had to start from scratch. In fact, this type of testing had never been done anywhere in the world.
To start the process, the team tested all of the MLG components to determine their compliance with design specifications. They were all found to be working as they should. Over several weeks, a full range of tests was carried out, in which the number 3 tire was spun at various speeds and imbalance weights to obtain data for computer modelling and vibration analysis.
During this testing process the team found that a high rotational imbalance had been created when the number 3 tire blew out and that the imbalance had been exacerbated on the landing attempt. The imbalance created vibration which was sufficient to overcome the system's various safeguards and eventually led to the collapse of the MLG.
It often takes a team of individuals with diverse technical backgrounds and expertise during investigations to resolve an incident. When everyone comes together with a common goal, the discoveries made can have a significant impact on safety. In this instance, the testing model of the landing gear was upgraded for the Bombardier Q400 fleet worldwide and enhanced the safety of this aircraft. Despite initial setbacks, all parties involved in the investigation gained knowledge and expertise from the findings. Ultimately, that is what makes the work of the TSB so rewarding.
An aircraft investigator for 16 years, Barry Holt is a TSB Regional Senior Technical Investigator in Edmonton, Alberta. He has been IIC on over 480 class 5 and 17 class 3 investigations in the Western Region, as well as being part the technical team for the investigation of the crash of a Sikorsky S92 off St. John's Newfoundland in 2009. Before joining the TSB, Barry spent many years in the field as an AME mostly on light and heavy helicopters, and as a Base Engineer for a remote mountain helicopter facility. He joined the Canadian Coast Guard on the west coast as the Senior Engineer and hoist operator on a Sikorsky S61N. Barry spends his non-work time helping on a cattle ranch and chasing cows on his horse, has a beautiful 12 year old daughter that he tries to keep up to, likes to shoot and hunt and is very busy on his acreage.
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