Archive for August, 2013

Full down autos

Saturday, August 17th, 2013

When practicing autorotations, the maneuver is initiated by reducing the engine to idle causing the freewheeling clutch to open, which then disconnects power to the rotor system. As the helicopter glides toward the ground, there are two ways to terminate the maneuver. One is to add the engine power back in and bring the helicopter to a hover, this is known as a power recovery autorotation. The other is to leave the engine at idle so the freewheeling clutch stays open, keeping the engine disconnected from the rotor system. Known as a full touch down autorotation, the pilot will increase collective pitch at the right time to create a momentary burst of lift to cushion the touch down.  In the helicopter industry, there are differing opinions on the value of practicing autorotations to the ground. 

The touch down requires precise timing because as the pilot adds collective pitch, rotor rpm begins to decay. If this is done too early, the rotor rpm can get too low causing controllability issues, excessive blade coning and loss of ability to cushion the touch down. By avoiding ground contact with a power recovery autorotation the risk of damaging the helicopter from a hard landing is reduced considerably. Some instructors and companies believe the risk of damaging a helicopter during touch down is too high and the benefit of actually landing does not justify the risk. The thought being that if a pilot performs the proper entry, maintains rotor rpm, maintains appropriate airspeed and then flares at the correct altitude the autorotation will be survivable. In reality, accidents from practice autorotations rarely cause serious injury or death, however, there have been many helicopters damaged from practicing autorotations. In fact, the US Army stopped practicing autorotations to the ground because they were damaging too many helicopters. 

I understand the risk vs. benefit analysis that leads to the decision to only perform power recovery autorotations. However, I think it is beneficial to practice autorotations in the most realistic environment that can be safely done, including full touch down to the ground. The risk can be minimized by using an experienced instructor with proper and extensive training in autorotations. Factory schools like Bell and Eurocopter have been doing full touchdown autorotations for many years with a good safety record.

Power limits

Thursday, August 1st, 2013

Helicopters powered by normally aspirated piston engines use manifold pressure as an indicator of power levels. Typically, pilots calculate limit manifold pressure for each day which is the maximum power setting allowed by the helicopter’s manufacturer. It is not necessarily the maximum rated horsepower limit for the engine. In many cases, the helicopter manufacturer de-rates the engine to reduce internal stress levels and extend TBOs. However, the pilot can exceed the limit manifold pressure (depending on factors like air density etc…) and still have available power.

In a gas turbine engine, the pilot must monitor three different indicators. Turbine outlet temperature (TOT) which refers to the temperature of the gas as it is exiting the engine, when the ambient air temperature is high this can be a limiting factor. Another is torque, which refers to the amount of torque the engine is applying to the transmission and is normally shown as a percentage. The third one is gas producer rpm, referred to as Ng or N1. When the air density is low, this section of the engine can reach its maximum operating rpm because it needs to spin faster to move the same amount of air.  A pilot of a turbine helicopter must monitor all three of these gauges and stop adding power when the first one reaches its limit.

Eurocopter uses something called a first limit indicator (FLI) to simplify the monitoring of all three parameters. One large gauge with a fixed yellow arc (indicating take-off power range) monitors all three parameters. So when the pilot adds power and the needle enters the yellow arc, then one of the three parameters has exceeded its maximum continuous power limit. To the right of the gauge, are the three values shown digitally (TOT, torque, Ng) and whichever one is the limiting value will be underlined in yellow.