Links to a Mishap
August 23, 2011
There we were one sunny Saturday in December on standby to help fight forest fires when we got the word to launch three aircraft. Everything was uneventful " or so we thought. Link No. 1 in the chain: the radar altimeter was written up on the aircraft as unreliable, which was a common occurrence at the time. A risk assessment performed by the crew determined it as an acceptable low risk since most of the aircraft had the problem. The crew would mitigate the risk by good crew coordination and ensuring the barometric altimeter was current and used for reference for height above the ground. The mission was executed, one flight of two and a single ship, en route to two separate locations.
Link No. 2 in the chain: During the first 45 minutes of the flight, the mishap aircraft experienced a hydraulic pump fault light, followed by excessive vibrations and increased temperatures. We executed a precautionary landing (PL) and began searching for the problem. It was determined that a hydraulic pump was failing and required replacement. The repair took most of the day as we waited for a part and completed the replacement. During that time, we received numerous calls wanting to know when the aircraft would be available to fight fires, as the fire situation was becoming critical and threatening civilian structures. The stress level and the desire to get into the firefight were beginning to rise. The crew completed the maintenance procedures and performed a maintenance run to ensure the system functioned as required.
The rush was on … Link No. 3. As the crew prepared for takeoff, we discussed required actions for arriving on the scene. Once there, we made contact with the overhead control aircraft that provided us a location to begin firefighting. The location was a large house, and the families were standing outside. Excitement was elevating. The mishap crew made a pass over the target to ascertain a good approach. In turn, we surveyed the departure path as well as a water drop point. Due to the winds and smoke, the approach path direction was limited and placed us over a set of large power lines. We performed a risk assessment and decided to remain high over the power lines and then descend to the drop point once clear. To mitigate further, we referenced the barometric altimeter and provided positive crew coordination to ensure wire clearance.
The radios suddenly got very busy as the mishap aircraft made six successful water drops. Due to the wind shift, we decided the seventh drop would be the last in this direction. Overhead control was in the process of handing off to a replacement crew and providing the handoff briefing over the radio, making aircraft internal communication difficult. This was Link No. 4.
Link No. 5: On approach for the final drop, the pilot not on the controls made a call stating the wires appeared closer than on previous runs. However, he took no other actions other than to direct the aircraft to the drop location. In a blink of an eye, the aircraft shook and began to climb. We first thought it was a heat riser when the crew chief (CE) on the load shouted, "We just hit wires and I punched the load!" That was followed by, "Clear up right!" from the right-side CE to get the aircraft clear of the fire zone and smoke.
Breaking the accident chain of events is easier than it might seem, because any chain is only as strong as its weakest link. The mishap chain was five links long, and breaking any one of those links could have prevented the accident. For the first link, we accepted the risk and implemented controls; however, we didn't effectively evaluate our controls during the mission and failed to make effective control adjustments.
While the second link was unexpected, a PL is a hazard that can be mitigated and controls put into place should the event occur, alleviating the need to rush.
The third link was a direct result of the PL and the desire to complete the mission, preventing clear thoughts to prevail to cancel the mission due to the amount of time it took to get back in the air.
The fourth link could have been broken by putting the intercommunication system pin switches down to improve internal aircraft communication.
Last, but not least, the fifth link could have been broken by the pilot not on the controls being more assertive and taking more aggressive actions when warning us about the power lines. A simple power adjustment could have been enough to clear the wires.
All of the links in this chain of events could have been covered through good composite risk management, which might have prevented the mishap and enabled the crew to complete the mission.