Archive for February, 2013

EMS helicopter pilots

Wednesday, February 27th, 2013

Having been an EMS helicopter pilot, I believe it is some of the most demanding flying a civilian pilot can do. The accident rate certainly supports this notion. One would think that this type of job would be at the top of the career ladder. One of those jobs that the most experienced and successful pilots would go after. However, that is not always the case.

Air medical should be an industry where turnover is low and getting in would take patience and persistence.  This environment seems to be more prevalent in corporate helicopter operations. One reason for this might be higher pay and benefits. Despite the demanding work an EMS helicopter pilot is required to do, pay and benefits are comparatively low. Would higher pay help the industry? Is pay and benefits the only issue that needs to be addressed? The debate on this subject seems to crop up a lot, especially the idea of raising compensation levels to help the safety problem. Not that any one individual pilot will fly any safer with a bigger paycheck, but industry turnover will certainly decrease.

I have known many good EMS helicopter pilots who have transitioned to fixed-wing aircraft or left the air medical industry to seek better pay and benefits. Instead of a stepping-stone to a higher paying job, EMS flying could be the career that pilots work hard to achieve. Over time, a low turnover rate will build an experience base of pilots skilled at making the tough decisions uniquely required by EMS flying.

A survey of pilots conducted by the National Emergency Medical Services Pilots Association found the number one suggestion to increase safety was to increase the quality and frequency of training. A close second was improving pilots’ salaries and benefits. Unfortunately, all of these strategies require increased funding at a time when cost pressures are high. However, overcoming these challenges and moving toward increased training and compensation would bode well for the air medical industry.

Rotor RPM

Wednesday, February 20th, 2013

Main rotor RPM is like airspeed to an airplane. It creates the airflow over the blades that produce lift. A rotor blade is a rotating airfoil that experiences a much higher airflow over the blade tips than the inboard areas. In order to improve the distribution of lift across the blades, engineers twist the blade so that the inboard part has a higher angle of attack for a given pitch angle. At a constant pitch angle, changing the RPM will vary the lift. However, in helicopter rotor design the main rotor RPM is a fixed value and lift is changed by varying the angle of attack by changing the blade’s pitch angle.

Main rotor RPM limits are established by the helicopter’s manufacturer. Normal operating RPM is shown on the RPM gauge as a green arc (the actual RPM will vary depending on rotor system design). Above the green arc is a yellow or caution arc that terminates at the rotor system’s maximum RPM red line. Rotor RPM that moves into in the yellow arc should be reduced by retarding the engine throttle or raising collective pitch to increase rotor drag. Allowing the rotor RPM to exceed the red line (an over speed) can increase the centrifugal forces to a level that can damage the rotor system. Depending on the severity of an over speed, an inspection or new part might be required.

Below the green arc is another yellow area with a minimum rotor RPM red line. Allowing the rotor RPM to decay into the yellow is recoverable, however going below the red line can become very dangerous. One way this can happen is if a pilot fails to lower the collective pitch (reducing the drag) quickly enough during an engine failure. FAA Part 27 certification requirements for autorotation require the manufacturer to demonstrate acceptable controllability and rotor RPM recovery at 5% below redline RPM. Rotor RPM allowed to drop more than 5% below red line might or might not be recoverable and will cause high coning and flapping angles coupled with significant vibrations. The rotor system can experience extreme stress levels which it was not designed for which will eventually lead to a failure of the hub or blade root. These types of accidents are always fatal.

Gas turbines

Thursday, February 7th, 2013

Gas turbine (jet) engines used in helicopters do not produce thrust. Instead, the air exiting the engine passes over a wheel (normally called the power turbine) with specially designed blades that turn a shaft. The shaft is geared down and connected to the transmission that drives the main rotor system. This design is called a turboshaft engine and its power is measured in shaft horsepower (shp). As in typical turbine engines, some of a turboshaft’s power is used to drive the inlet compressor or gas producer section. 

Eurocopter’s AS350 series helicopter uses Turbomeca’s Arriel line of engines. The Arriel 1B was certified in 1977 with 640shp. Throughout the years various upgrades have raised the output power, the most recent was in 2011 with the Arriel 2D at 951shp. The Arriel design uses a two stage compressor. The first stage is an axial compressor that draws in ambient air and increases its pressure and speed. It is then directed to the centrifugal compressor that further compresses the intake air to 118.9 PSI and raises its temperature to 335 degrees C before the air enters the combustion chamber. Because the centrifugal compressor is designed to be very efficient at high turbine speeds (high power demand) a bleed valve vents the excess pressure from the axial compressor at low turbine speeds (low power demand). 

The bleed valve is normally open when the engine is shutdown, during starting, and at low power settings. Unlike some compressor bleed valves the Arriel series engines’ are modulated, so as the pilot increases the power the bleed valve gradually starts closing. In the Eurocopter AS350 helicopter when the bleed valve is fully closed a green and white indicator in the cockpit disappears.  If the indicator does not disappear at high power settings, this tells the pilot that the bleed valve has failed to close and maximum engine power will not be available. If the indicator does not reappear at low power settings, the bleed valve has failed to open and the engine may experience compressor stall or surge.  In this case, the pilot should avoid abrupt power changes.