Archive for January, 2010

Low-G pushovers

Friday, January 29th, 2010

A two-blade or semi-rigid rotor system (such as the Robinson or some Bell series helicopters) is susceptible to a phenomenon called mast bumping. To avoid mast bumping it is important to fully understand the limitations and performance capability of this type of rotor system.

In order to produce thrust a helicopter’s rotor system must be loaded. Controlled by the cyclic, the swash plate changes the pitch angle on each blade separately. This creates an imbalance of thrust across the rotor disc forcing the disc to tilt, which causes the helicopter to roll or pitch in the desired direction.

Pushing the cyclic forward following a rapid climb or even in level flight places the helicopter in a low G (feeling of weightlessness) flight condition. In this unloaded condition rotor thrust is reduced and the helicopter is nose low and tail high. With the tail rotor now above the helicopter’s center of mass, the tail rotor thrust applies a right rolling moment to the fuselage (in a counter-clockwise turning rotor system). This moment causes the fuselage to roll right and the instinctive reaction is to counter it with left cyclic. However, with no rotor thrust there is no lateral control available to stop the right roll and the rotor hub can contact the mast. If contact is severe enough it will result in a mast failure and/or blade contact with the fuselage.

In order to recover the rotor must be reloaded before left cyclic will stop the right roll. To reload the rotor immediately apply gentle aft cyclic and when the weightless feeling stops, use lateral cyclic to correct the right roll.

The best practice is to exercise caution when in turbulent air and always use great care to avoid putting the helicopter in a low-G condition.

Safer night ops

Tuesday, January 19th, 2010

Threats, clearly visible during the day, are masked by darkness. In fact, controlled flight into terrain (CFIT) at night is a major problem for rotor-wing operations. CFIT is defined as colliding with the Earth or a man-made object under the command of a qualified flight crew with an airworthy aircraft.

During the 1970s, CFIT became a major problem for commercial aviation. In response the FAA mandated the installation of ground proximity warning systems (GPWS) in commercial airliners. Although this resulted in a drop in CFIT accidents, these earlier systems were plagued with false and late warnings. Improved versions, called enhanced ground proximity warning systems (EGPWS), were introduced. These systems have made a valuable contribution to the reduction of fixed-wing CFIT accidents.

CFIT at night during VMC has been especially troublesome for helicopters in the air medical industry. According to the Air Medical Physician Association, half of all EMS accidents happen at night. EGPWS have been discussed as a solution to reduce the air medical helicopter accident rate. However, because of the unique low-flying operation of helicopters the effectiveness of current EGPWS is unclear. This prompted Honeywell to introduce the Mark XXII EGPWS, specifically designed to address the needs of helicopters. Moreover, the company is developing a database of power lines to add to the system. As computer memory capability grows, databases will be able to contain more detailed maps.

However, by the time the EGPWS activates, the pilot has probably already lost situational awareness. A method to help with situational awareness is improving the pilot’s ability to see obstructions at night. That’s the technology behind night vision goggles (NVG). They work by detecting and amplifying existing visible light, so there must be at least some light available for them to work. Originally NVG were only for military use, but recently they have been allowed in the air medical industry, and more than half of the EMS helicopters are flying with them.

Another technology that holds promise is enhanced vision systems (EVS) which detects and displays thermal energy not visible to the naked eye. In this arrangement a camera is mounted in the nose and feeds the image to a monitor in the cockpit. Some glass cockpit systems will project the image behind the attitude indicator for better situational awareness. These systems are effective in smog, smoke, duststorms, and other limited visibility situations. Likewise, they can help in brownout and whiteout conditions. The U.S. military uses thermal imaging systems in combination with NVGs.

The air medical industry is expecting the FAA to possibly mandate additional equipment requirements like they did with earlier with commercial aviation. With the different technologies available it will be interesting to see what happens.

Servo transparency

Friday, January 8th, 2010

Pilots who learn to fly in smaller helicopters probably hear very little about servo transparency, yet this phenomenon has caused or played a role in several accidents. When giving flight reviews I have found some helicopter pilots who totally misunderstand why and how it happens. However, the concept is not too difficult to understand.

Because of the higher control forces in larger helicopters, hydraulically boosted servo actuators are used to assist the flight controls. The maximum force that these servo actuators can produce is constant and is a function of hydraulic pressure and servo characteristics. Engineers design the hydraulic system to adequately handle all aerodynamic forces required during approved maneuvers. Even so, with certain aggressive maneuvering it is possible for the aerodynamic forces in the rotor system to exceed the maximum force produced by the servo actuators. At this point, the force required to move the flight controls becomes relatively high and could give an unaware pilot the impression that the controls are jammed. To prevent servo transparency, pilots should avoid abrupt and aggressive maneuvering with combinations of high airspeed, high collective pitch, high gross weight, and high-density altitude.

The good news is that this phenomenon occurs smoothly, and can be managed properly if the pilot anticipates it during an abrupt or high-G load maneuver. On clockwise-turning main rotor systems the right servo receives the highest load, so servo transparency produces an un-commanded right and aft cyclic movement accompanied by down collective. The pilot should follow (not fight) the control movement and allow the collective pitch to decrease while monitoring rotor rpm, especially at very low collective pitch settings. The objective is to reduce the overall load on the main rotor system. It normally takes about two seconds for the load to ease and hydraulic assistance to be restored. However, be aware that if the pilot is fighting the controls when this happens, the force being applied to the controls could result in an abrupt undesired opposite control movement.

Many of these accidents have happened while aggressively flying the helicopter at low altitudes, leaving very little time to recover. Most important for avoiding this kind of accident is to follow the aircraft limitations published in the helicopter’s flight manual.