Archive for October, 2012

Running landings

Wednesday, October 24th, 2012

A running landing is used when there might not be enough available power to hover. This maneuver is typically used at high gross weight, high density altitude and for some emergency procedures involving the tail rotor. A twin engine helicopter might use this type of landing when making a single engine landing. As such, pilots normally practice this type of landing.

Performing the maneuver is fairly straight forward. The pilot would typically use a shallower approach; align the helicopter with the touchdown area and touchdown at or above effective translational lift. Depending on the reason, the touchdown airspeed is normally between 20 and 40 knots, however, it should be at the lowest airspeed for the situation to minimize ground run. As the helicopter slides to a stop the pilot should use the cyclic control to maintain ground track, the pedals for heading (important not to allow the helicopter to slide sideways), and the collective control to apply braking force.

When practicing this maneuver, extreme caution should be taken to insure a level and unobstructed landing surface. The follow NTSB account illustrates just what can go wrong even when care is taken.

On May 24, 2011, at 1503 eastern daylight time, a Schweizer 269C, N7505Y, sustained substantial damage during a practice run-on landing at Asheville Regional Airport (AVL), Asheville, North Carolina. The certificated flight instructor (CFI) and private pilot receiving instruction were not injured.

According to the pilot receiving instruction, who was also the owner of the helicopter, the purpose of the flight was to conduct a flight review. Approximately 50 minutes into the flight, the CFI asked the pilot to demonstrate a run-on landing to runway 16. The pilot conducted the approach for landing at about 40 knots and touched down left of the runway centerline on both skids. As he lowered the collective, the helicopter’s right skid contacted a runway centerline light, shearing off the right skid and its support arms.

The pilot raised the collective, picked the helicopter up to a hover and turned towards the taxiway. Shortly after, the engine and rotor RPM began to drop, the pilot opened the throttle and lowered the collective, setting the helicopter on the left skid. The helicopter rolled over and came to rest on its right side, resulting in substantial damage to the main rotor blades.

A post-accident examination by the pilot revealed that during the right skid’s impact with the centerline light the front landing gear crossbeam was pushed aft, crimping the fuel supply line.

The pilot later made these comments:

“From the skid tube marks on the runway, it became evident that the right, central skid shoe (attached to the bottom of the skid) had contacted the recessed centerline runway light straight-on. As such, the shoe jammed firmly against the flat surface of the runway light housing at the lens face of the light (north end of the light housing). Here is my main concern: Had my run-on landing been a true emergency landing without the benefit of power (that was instantly available after impact, though short-lived), the outcome would have been much different. The right under-carriage, now minus all skid landing gear, would have contacted the asphalt surface spinning us clock-wise, the left skid would have dug into the surface, and we would have cart-wheeled down runway 16 with a certain likelihood of serious injury or death. The threat of a helicopter fire would have been very likely.

No one would expect a 1.5 inch wide skid shoe would ever jam into an approximately 2 inch wide light port on this recessed light. But it did, and my helicopter is destroyed as a result.”

Rotor blade icing

Friday, October 12th, 2012

Flying in conditions conducive to ice formation is problematic for virtually all helicopters. Moreover, many twin engine IFR helicopters are not certified for flight in known icing conditions. As such, helicopter pilots should understand the problems an encounter with icing can create for the rotor system.

Ice buildup on rotor blades will change the shape of the airfoil and consequentially, its ability to produce lift while increasing drag. The increased drag will slow the main rotor requiring the pilot to add power – which in some cases might not be available. Ice accumulation on the airframe can increase the helicopter’s gross weight requiring more power as well. Ice buildup is rarely, if ever, symmetrical causing an imbalance that produces vibrations in the rotor system. These vibrations can cause shedding of the ice and if all the ice comes off, vibration levels, lift and drag will return to normal. Asymmetrical shedding, however, can make the vibrations worse. Hopefully, the increased vibration will shed the remaining ice before any damage can occur. Ice accumulation is less on the outboard section of the rotor blade which is helpful because this area produces a larger amount of lift. However, an autorotation could be more difficult as the driving region is closer to the blade’s center.

Deicing refers to removing ice that has accumulated, while anti-icing is the prevention of ice formation. The few helicopters that having ice protection on the main rotor system use a de-icing system as the power required to anti-ice a main rotor system is extremely high. One of these is the Sikorsky S92 and it uses heater mats in the rotor blades to melt a thin layer of ice in contact with the blade surface causing the remaining ice to shed from the blade. According to Sikorsky, heat is applied to the mats to melt the ice in specific zones at precisely the right time for controlled shedding. Opposite main rotor blades are deiced simultaneously in order to prevent rotor imbalance and small sections of the rotor blades are deiced alternately to reducing the amount of electrical power required at any given time. The tail rotor ice protection system can be set to de-icing mode, which applies power in a scheduled manner or anti-icing mode in which heat is continuously applied to tail rotor heating mats.

Slope landings

Thursday, October 4th, 2012

Not every surface a helicopter lands on is perfectly level. So a slope landing is a maneuver that helicopter pilots need to know how to perform. The first step is bringing the helicopter to a stabilized hover into the wind and insure the ground is stable (for example, no loose gravel). Care must be taken when making pedal turns to avoid getting the tail rotor too close to ground. In the case of the ground sloping laterally, the pilot should slowly lower the collective until the upslope skid contacts the ground. At this point, apply lateral cyclic to firmly seat the skid into the slope. Maintain heading control with the pedals to prevent the skid from pivoting. Holding the upslope skid against the slope with cyclic, continue slowly lowering the other skid a little at a time with the collective. As the pilot continues lowering the collective, more lateral cyclic is required to hold the upslope skid firmly against the ground. If the pilot runs out of lateral cyclic prior to the downslope skid becoming firmly seated on the ground, then the slope is too steep and the landing should be aborted. When performing slope landings pilots need to be aware of the increased risk of dynamic roll over and, with a semi-rigid rotor system, mast bumping.

Once both skids are securely down, some instructors recommend centering the cyclic after the collective reaches flat pitch in order to have more clearance with the rotor system on the upslope side, others recommend keeping it displaced into the slope for the duration of the landing to prevent any sliding.

Lifting off a slope is essentially the reverse procedure. Raising the downslope skid with collective while moving the cyclic back neutral. Once the helicopter is level, lift off the slope. The pilot should keep in mind if a lot of weight is off loaded the CG might have changed enough to shift the cyclic neutral point, which could compromise a safe lift off.