The Art of the Helicopter John Watkinson - Karatunov.net

Engines and transmissions 237
Sensing the collective lever velocity is also useful in the case of a rejected landing. This
is where the machine is landed but the ground is found to be unsuitable. Perhaps the
machine has started to break through a crust of ice on top of snow, or the landing area
turns out to be waterlogged. Ordinarily the reduction of collective at touchdown would
cause a slight overspeed causing the gas generator to spool down. Pulling collective to
reject the landing would cause droop. Sensing the collective velocity would help the
fuel control anticipate the power requirement.
In twin-engine machines, it is necessary to adjust the fuelling in such a way that the
engines share the load. If the FADEC system is sensing TOT and N1, either of these
can be used to achieve a load share. In practice, engine tolerances will mean that a
balance of TOT may result in a small difference between N1 and vice versa.
Mechanical fuel controls and correlators are complex and expensive and prone to
wear. The substitution of stable digital electronics will result in the elimination of wear
and allows arbitrarily complex transfer functions to be implemented easily. However,
the downside of FADEC is that it requires the control software to be utterly reliable.
The difficulty with computers is that the number of states and combinations that can
be achieved is staggering and it is therefore very difficult to prove that the computer
can correctly recover from every possible state. In the development of FADEC systems
there have been some spectacular failures, including an in-flight rotor speed runaway
on a Chinook that required the pilot to climb at record speed to prevent the rotors
being thrown off until the engines could be shut down.
In well-engineered systems a FADEC failure should leave the pilot, or co-pilot, with
some form of manual power control, even if RRPM is not as precise.
6.25 Turbine instruments
As there is no throttle an inlet pressure gauge would be pointless. The actual power
being generated is a fairly stable function of the reading displayed on the fuel flow meter
so this or the torque meter may be used for power checks. An intake air temperature
indicator will be provided so that the pilot will have an indication of the maximum
power available. N2 (power turbine RPM) is proportional to RRPM and needs no
separate indication. However, N1 (gas generator RPM) will have its own tachometer.
The power turbine inlet temperature (PTIT) is measured by thermocouple in the engine
and displayed on the instrument panel and will be red-lined at a safe temperature above
which the turbine blades may be damaged. In some cases the turbine outlet temperature
may be measured instead. This is simply because the temperature is lower at the outlet
and the sensors may be less expensive. In the case of an engine failure, the remaining
engine may run at higher power and this will cause PTIT/PTOT to exceed the red line
and enter the contingency power range. This will result in a caution light illuminating.
Usually a timer also runs to assess the stress caused to the engine. The oil pressure and
temperature are displayed as for the piston engine. Oil quantity may also be displayed.
There may also be chip detectors in the oil system.
6.26 Fuel management
When refuelling, whoever actually undertakes the task, the responsibility is taken by
the commander of the machine. It is essential to specify the quantity and type of fuel
required and to check that this has actually been supplied. AVTUR and AVGAS smell