TSB: Automation Played Role in Sikorsky S-92 Incident
In its report on a 2011 incident in which a Sikorsky S-92 nearly crashed off the Canadian coast, the Transportation Safety Board (TSB) of Canada implicates the pilots’ poor understanding of automation, insufficient basic flying skills and a misleading flight manual, which it says caused an inadvertent, vertiginous descent.
In its report on the incident, the TSB expresses concern about erosion of manual flying skills. When the operator places too much trust in automation, according to the report, “Pilots may not be gathering the information normally required when manually flying a helicopter.” As a result, if a problem arises, pilots can easily find themselves out of the loop. In addition to the extra time they might require to figure out what is happening, pilots may lack hands-on experience dealing with the predicament they find themselves in, the investigators said.
The S-92, operated by Cougar Helicopters, took off from the Sea Rose production vessel in the North Atlantic at 2:57 p.m. on June 23. One minute and 26 seconds later, the aircraft was hovering, unexpectedly, just 38 feet above the surface, with the rotor downwash kicking up sea spray covering the windows. In the course of less than a minute-and-a-half, it had climbed into the 200-foot cloud base to an altitude of 541 feet and had then descended in a nose-high attitude and at a vertical speed that reached 1,880 feet per minute (fpm). No injuries were reported among the five passengers and two pilots.
The pilot flying was the captain, in the right seat. A standard rig departure consists of a vertical takeoff to 30 feet above the deck, and then a 10-degree nose-down attitude to accelerate to the takeoff safety speed (Vtoss), the TSB explained. As the helicopter’s airspeed reaches Vtoss, the pilot increases the attitude of the helicopter to approximately 5 degrees nose-up and continues to climb out at Vtoss. At the same time, the pilot monitoring (in this case, the first officer) raises the landing gear. At that point, the pilot flying engages the desired autopilot mode, such as the go-around (GA) mode.
Cougar Helicopters uses automation as much as possible to maximize safety and passenger comfort, investigators learned, a policy shaped by an earlier incident in which the pilots encountered control problems during a manually flown IMC departure.
Changes to Aircraft Manual and Training Requirements
The St. John’s, Newfoundland-based operator thus encourages its pilots to use the GA mode for offshore rig departures, and this policy conforms with the S-92’s flight manual. When engaged, the GA mode will command a wings-level climb. The collective axis will capture a 750-fpm vertical climb, the pitch axis will capture 80 knots and the roll axis will maintain the present heading.
The S-92 captain wanted to follow the GA procedure. However, just before engaging the GA mode, he made a large aft cyclic input. It was inadvertently larger and more rapid than normal, investigator-in-charge Daryl Collins told AIN. So when the pilot engaged the GA mode, the pitch attitude was not stable and was increasing through 2.4 degrees nose-up, causing the helicopter to enter a nose-high, decelerating pitch attitude.
Before he engaged the GA mode, the captain did not feel any pressure on the cyclic. He was not concerned about attitude because he believed the helicopter would adopt the usual climb-out profile after he engaged the GA mode. It did not. Worse, the captain made another abrupt aft-cyclic input while the airspeed was already decreasing.
Once the airspeed had decayed below Vmini, the autopilot decoupled and alerts sounded. Displaying a reticence commonplace among junior pilots, the first officer was reluctant to take control from the captain. He did make attitude and airspeed calls and believed the captain was taking correcting action. However, he was not, because he was subtly incapacitated.
Pitch during the descent went as high as 23 degrees. Airspeed dropped as low as 25 knots. The captain eventually recovered by slowly lowering the nose and, after breaking out of the cloudbase, increasing collective pitch.
The TSB report says that the crew’s first error was engaging the GA mode without having first stabilized the aircraft’s attitude. The flight manual did mention that the GA mode should be engaged during trimmed, steady-state flight. A month after the incident Cougar Helicopters published a memo stating “prior to engaging any autopilot function, the pilot must not apply any pressure to the collective or the cyclic. Any pressure will cause an error in the system and result in unanticipated aircraft attitudes.”
However, the TSB deemed the flight manual “misleading” because other than the requirement to be above Vmini (the minimum control speed in IMC, 50 knots), “There were no other flight envelope restrictions outlined.” The manual indicated that the GA mode could be used to recover from an unusual attitude. In April this year Sikorsky made an addition to the flight manual: “when selecting GA, ensure the aircraft is in a stable climb and cyclic is trimmed to zero force.”
To address concerns about basic flying skills, Cougar Helicopters’ pilots are now required to conduct at least two manually flown approaches to minimums every 90 days. The company has also changed its policy for unusual-attitude recovery by issuing a clarified, manually flown procedure and enhanced simulator training.