NAME WITHHELD BY REQUEST
Cargo helicopter pilots look forward to performing external load operations that challenge their abilities. As exciting and fulfilling as it is to accomplish a mission, there is the inherent risk of having to jettison a heavy external load in the event of an emergency. During training recently, a CH-47F crew jettisoned a 10,000-pound external load on a large installation.
Preflight
Preflight is conducted prior to every flight in every unit in the Army. These inspections are imperative to the safe operation of all aircraft. This crew consisted of an experienced pilot in command (PC), a pilot (PI) that was in PC progression, a flight engineer with years of experience and a crew chief that was very familiar with external load operations. The crew conducted a thorough preflight and inspected the cargo hooks prior to executing the training mission. Everything appeared to be functional.
Run-up
During run-up, the crewmembers did the appropriate cargo hook checks and all functions were operational. The pilots checked their hook release buttons and the hook opened appropriately. The crew chief checked his hook buttons, which also worked as advertised. All checks of the hooks actuated the hook when they were pressed. When the hook was in the OFF/SAFE position, it would not open when the buttons were pressed, as it shouldn’t.
Mission execution
The crew flew to the training area and selected the 10,000-pound block for the operation. They already had a performance planning card (PPC) which had supporting data showing the helicopter would not exceed any limitations while performing the external load training. The crew then briefed the actions to be performed by each position in the aircraft for normal operations and in the event of an emergency, such as an engine failure.
They performed a few iterations of elevator drills (when a helicopter crew hooks up the sling load and sets it down in the same spot without flying to a new location). Once the drills were complete, they briefed their next maneuver, which was flying a traffic pattern after hooking up the external load. After takeoff, the PC announced they were above single-engine airspeed and 250 feet above the highest obstacle, which meant it was time to “safe” the cargo hook by placing it into the OFF position. This is done to prevent inadvertently releasing the load in flight by one of the cargo release buttons.
Hook failure
The crew flew the first pattern without incident. While in a right-hand turn on the second pattern, however, the load jettisoned from the hook, which the crew chief announced. Immediately, the PC asked for confirmation that the external load landed in a safe location. The crew chief responded that the load was safely on the ground. The PC then verified the hook was still in the safe position, which it was, followed by a question to the crew chief asking if the load was manually released. The crew chief responded that the load was not manually released.
On the ground
The PC decided to land near the sling load to inspect the aircraft and hook to see what went wrong. Once on the ground and shut down, the crew called their command to notify them of what happened. Upon a visual inspection of the hook, everything looked normal and appeared to function as it should.
Findings
After the aircraft was cleared to fly back to the airfield, maintenance personnel began inspections and checked the operation of the cargo hook. It was determined the load jettison was caused by a mechanical failure of the hook and was not the crew’s fault. The cable leading to the hook from the emergency jettison lever was faulty. The cable had come out of its retaining sheath, which took up any slack that was required to allow the hook to swing while under load. During the right-hand turn, the hook swung with the weight of the load, which allowed the cable to tighten as if the emergency jettison handle had been pulled, which released the load during flight.
The outcome
Upon further investigation into the fleet, multiple CH-47Fs had similar issues with the cargo hooks. The jettison cables were starting to show the same flaw as the one that inadvertently released the external load. A precautionary maintenance message was sent out across the fleet to notify other units of this issue and what needed to be done to correct it. Fortunately, in this mishap, the helicopter was conducting training over an improved area and was well clear of any civilians or service members, allowing for a safe spot for the external load to land.