Archive for August, 2011

Hot refueling

Monday, August 22nd, 2011

Many times to save time or a start cycle on a turbine engine, pilots and operators will perform hot refueling, or what is technically called Helicopter Rapid Refueling (HRR). The National Fire Protection Association (NFPA) publishes document 407 titled “Standard for Aircraft Fuel Servicing” which includes a section on HRR.

According to the NFPA only turbine engine helicopters fueled with Jet A or Jet A-1 fuels shall be permitted to be fueled while an onboard engine is operating. All sources of ignition must be located above the fuel inlet port(s), vents or tank openings. Ignition sources include engines, exhausts, APUs and combustion-type cabin heater exhausts. 

Some additional NFPA requirements for HRR are: 

  1. An FAA-licensed helicopter pilot shall be at the aircraft controls during the entire fuel servicing process. 
  2. Passengers shall de-board to a safe location prior to rapid refueling operations. 
  3. Passengers shall not board or de-board during rapid refueling operations. 
  4. Only designated personnel, properly trained in rapid refueling operations, shall operate the equipment. 
  5. All doors, windows, and access points allowing entry to the interior of the helicopter that are adjacent to, or in the immediate vicinity of, the fuel inlet ports shall be closed and shall remain closed during refueling operations. 
  6. Fuel shall be dispensed into an open port from approved dead-man type nozzles, with a flow rate not to exceed 60 gpm or it shall be dispensed through close-coupled pressure fueling ports. 
  7. When fuel is dispensed from fixed piping systems the hose cabinet shall not extend into the rotor space. 
  8. A curb or other approved barrier shall be provided to restrict the fuel servicing vehicle from coming closer than 10 ft from any helicopter rotating components. If a curb or approved barrier cannot be provided, fuel servicing vehicles shall be kept 20 ft away from any helicopter rotating components and a trained person shall direct the fuel servicing vehicle’s approach and departure. 

Even with these safety precautions I have talked to operators that will not hot refuel because of the increased risk.

Retreating blade stall

Tuesday, August 9th, 2011

Flying a rotor system edgewise through the air creates a problem known as dissymmetry of lift. One side of the disc advances into the wind (headwind) while the other side is retreating (tailwind). For a fixed angle of attack, the lift on the advancing side is greater creating a lift imbalance that increases with airspeed. The rotor system equalizes lift by flapping.

How flapping works is by changing the angle of attack in response to the varying airspeeds the blade encounters as it moves around the rotor disc. When the advancing blade experiences a higher airspeed, the lift on that blade increases forcing it to move up. This upward movement changes the direction of the blade’s relative wind reducing its angle of attack. On the retreating side just the opposite happens. The reduced airspeed causes a decrease in lift causing the blade to move down, increasing its angle of attack. You can only increase an airfoil’s angle of attack so much before it stalls. As the helicopter continues to fly faster the retreating side must continue to increase its angle of attack to compensate. At some airspeed, the retreating blade begins to stall.

From the pilot’s perspective, when this happens an abnormal vibration will be felt, the nose can pitch up, and the helicopter can have a tendency to roll in the direction of the stalled side. The amount and severity of pitch and roll will vary depending on the rotor system design.

The tendency for the nose to pitch up is because the spinning rotor system acts like a gyroscope and therefore experiences gyroscopic precession (a physical property that states when an external force is applied to a rotating body the effect will happen approximately 90 degrees later in the direction of rotation). As such, when the retreating blade stalls and stops producing lift, the effect of this happens toward the rear of the rotor disc. This causes the disc to tilt back, and the nose to pitch up. The pilot should lower the collective pitch first and then reduce forward airspeed to recover.

Conditions like high density altitude, steep or abrupt turns, high blade loading (caused by high gross weight), turbulent air and low rotor rpm will increase the likelihood of encountering retreating blade stall when operating close to a helicopter’s Vne (never exceed speed).