Saturday, November 1, 2014
Check Your Performance Charts
|These Diamond J, Inc. torque and turbine outlet temperature gauges installed in a Bell 206B3
catch everything, including this 103 percent for one second torque. Photo by Mark Colborn
So, why do 25 percent of all accident pilots keep driving their machines with perfectly good engines into the ground? Imagine driving your car up a mountain pass where the air gets progressively thinner. You press down on the accelerator pedal to maintain the same speed but as the mountain gets steeper, you have to press down farther on the pedal. At some point, the pedal is on the floor and the car slows down. Your car’s engine is providing as much power as it possibly can. In a helicopter going over the same mountain pass, you can continue to pull up on the collective and try to demand more power from the engine, but this will only increase the rotor blade angle of attack (or drag), and reduce rotor RPM. With any decrease in rotor RPM there will be a reduction in the effective disc area due to an increase in the coning angle. This situation is terminal and is referred to as “overpitching.” At a certain engine and rotor RPM, depending on your helicopter model, the machine will simply stop flying and you will descend out of control.
Helicopters perform much differently at higher altitudes, high outside air temperatures, and at or near maximum gross weight. As a helicopter climbs higher, the air gets thinner and less lift is produced by the rotor blades at a given power setting. The decrease in performance can be so significant that the pilot may not realize he or she is out of power until final approach, which is far too late. An approach angle and subsequent collective input at the bottom of an approach that worked just fine during a solo flight, or at a lower density altitude, suddenly won’t arrest the helicopter’s rate of descent.
In a 2006 U.S. helicopter accident, a commercial pilot and owner of an OH-58C approaching his ranch for landing stated he “lifted up on the collective to add power [but] no power was present.” He then lowered the collective and nose to gain forward airspeed. The pilot wrote in his statement that he “elected to keep forward speed to the ground so impact with the ground would be at an angle [glancing blow] versus straight down.” The National Transportation Safety Board’s (NTSB) probable cause listed the accident as a loss of engine power for reasons undetermined, but a contributing factor was the low altitude and rough uneven terrain. This was a “power available” vs. “power required” problem. In essence, the pilot demanded more power than the engine could deliver, drooping the rotor.
In another 2006 U.S., helicopter accident, a pilot was on a mission photographing raptors in a box canyon. He was hit by a strong downdraft from a forecasted frontal passage, and subsequently lost control and crashed. The pilot provided a recommendation on how the accident could have been prevented by stating in the NTSB Owner/Operator Report of Accident: “A helicopter with more horsepower may have been able to recover from the burst of turbulence on the early leading edge of the approaching front.” There is no mention in the record of the aircraft’s performance data, capabilities, or environmental conditions (other than temperature and the early arrival of a forecasted frontal passage) at the time of the crash. It should also be noted that the crash site was at 4,700 ft. MSL. The NTSB’s probable cause concluded: “The pilot’s failure to maintain aircraft control while maneuvering resulting in an impact with terrain. Contributing factors were the gusty winds, downdrafts, and box canyon.” The NTSB’s probable cause would tend to support the pilot’s naive view that his only mistake was not using a bigger, more powerful helicopter.
On May 8, 2011, the pilot of a Robinson R44 was contracted to transport two photographers over a downhill skateboard competition near Golden, Colo. The pilot, an instructor with over 2,000 hours, topped off the fuel tanks and flew straight to the mountain with the blessing of the company owner, but with one caveat; “Whatever you do, don’t slow down.” Several videos from different angles (available at: http://www.youtube.com/watch?v=vlg8xzuJDnY) show the helicopter circle in over the competition and slow down. It then descended, the rotor blades clipped a tree, the helicopter hit the hillside and then rotated forward on the toes of the skids coming to rest on the cockpit’s nose. The videos also indicate that the rotor blades were coning significantly, and the engine RPMs sounded like they were decreasing. According to the NTSB, the pilot estimated that the helicopter’s out-of-ground effect hover capability at its approximate weight to be around 5,000 ft. pressure altitude. The density altitude at the crash site was calculated to be 10,600 ft. This accident is a dramatic example of a complete lapse of pilot judgment, a lack of situational awareness, and a “power available” vs. “power required” loss of control accident. The pilot never should have attempted to decelerate through effective translational lift.
Why do pilots continually put themselves into a position where they run out of power or exceed their machine’s limitations? Perhaps it’s the training they receive, or lack thereof. Most flight training occurs in a controlled environment, either at or near sea level altitudes, at a low gross weight, and with minimum fuel states. And unless they receive advanced mission training for hot, high, or heavily loaded conditions, like most military pilots receive as a part of advanced flight training, they are not properly prepared to operate at the limits of the helicopter’s performance envelope. Since many pilots have never operated their machines in these extreme conditions, when they encounter power issues they react inappropriately or incorrectly. The results are generally tragic, if not deadly.
Thorough preflight planning is essential to preventing loss-of-control accidents. Flight manual performance chart calculations are an integral part of this planning. However, charts vary wildly across manufacturer models. Military flight manuals are generally chocked full of performance charts, and it is easy to find one for a particular operation at a certain pressure altitude, temperature and weight. Some manuals are not as detailed as others, and may require pilot interpretation of the available charts to match a planned flight environment.
Density altitude represents the combined effect of pressure altitude and temperature. At any altitude, warmer air is less dense than colder air. If operating at a mountain location and the temperature is high, your density altitude could be significantly higher than your actual pressure altitude. A high density altitude will reduce your hover ceiling, limiting landing sites; reduce operating margins by limiting payloads; and can significantly reduce your rate of climb speed, limiting takeoff and landing clearances.
It is helpful to know the actual density altitude at your operation location, but to find out if you can actually operate at any particular altitude, let the performance graph find the answer for you by entering the chart at the correct pressure altitude and temperature lines. Expect ambient conditions, however, at the intended area of operations, to be different from those planned for. Flight manual performance graphs denote the operation of a brand new or “spec” engine with clean rotor blades, so calculated values must always be verified with an actual power check under the ambient conditions that exist at an operating site. Pilots, after loading or reloading passengers and cargo, would be well advised to re-verify those power checks each time before attempting another takeoff.
Pilots must also consider other items that may affect the performance of their helicopter. Are they operating the heater or air conditioner? Is the engine anti-ice in the “ON” position? Are the doors open or off? Optional equipment can significantly affect drag and throw off performance calculations.
Correct performance calculations depend on getting an accurate and current weight for your helicopter. In the case of the Sikorsky S-61 – (callsign “Iron 44”) – that crashed into trees while attempting to takeoff from a mountain firefighting heli-base in Redding, Calif. in 1988, company officials understated the weight of the helicopter in order to secure a U.S. Forest Service contract. Because the heavy lift machine subsequently burned, investigators were never able to definitively figure the actual weight, but suspected it was considerably more than reported.
So why is obtaining a dependable and accurate weight crucial to all subsequent performance calculations? Helicopter engines are so powerful and dependable, that you can easily reach and exceed an airframe structural weight limit long before you reach an environmental power limit. Repeated overloading of an airframe can cause structural damage or a premature failure of rotor masts, drive shafts or drive links. The UH-60 Black Hawk, under certain environmental conditions for instance, is more than powerful enough to lift more weight than the cargo hook is designed for. Repeated overloading of the hook has actually bent airframes in the past, permanently grounding machines.
Research and analysis by the IHST identifies that loss-of-control accidents are predominantly related to human factors. In most cases, accidents were caused by failure to perform specific procedures, execute a proper decision, communicate, or adequately plan. Training and safety management are the primary recommendations for intervention in this case, according to the IHST analysis. Training must include extensive discussions of aeronautical knowledge relating to piloting skills, airframe capabilities, proper power management, and specific information regarding mission profiles (i.e., what you do and where you do it). All recommendations center on the integration of safety and operations management.
Except for a recent accident where a moose attacked a hovering helicopter, pilots haven’t figured out any new ways to crash a helicopter! If we are going to significantly reduce the number of accidents in our industry, we have got to do a better job of training our new pilots and cross-referencing knowledge with evidence of these problems. Logically, to prevent loss-of-control accidents due to performance planning issues, we need to start training like we will be, or could be, tasked to fly. With the widespread availability of video cameras, pilot mistakes can more often be seen. In years past, we could only read about these accidents and imagine how events transpired, but when it is captured on video like the intrepid downhill skater photography ship, the three- to four-second mystery of how the helicopter crashed is removed and the impact as a training tool is profound. Perhaps someday we will see a small personal video camera in every cockpit.