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Bell 47 engine, engine mount, engine controls & cooling fan

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E:TIP 1: 47G3B1: induction hose:

Check that the induction hose is not creased inside the clamp on the top of the engine basket at the rear of the transmission where it turns thru nearly 90 degrees. If it is, it may "suck in" and restrict the flow of air to the air filter in the wrong place at the wrong time!


E:TIP 2: all: Carb heat:

If at any time in flight you feel that “something is not right” with the engine – your first action, provided that it is safe to do so (height, power requirements etc), would be to pull the Carb heat full on. This ensures that any potential Carb ice problems are eliminated, PLUS gives you an alternative filtered Carb air source should the problem be with the main air induction hose. On normally aspirated models, use Carb heat whenever indicted by the Carb temp gauge. The engine will stop without warning if it picks up ice.


E:TIP 3: all: Power loss when hot:

If at any time in flight you have an unexplained loss of power that is of a magnitude that, though the engine keeps running, an emergency landing has to be made, it may be a problem with an inlet valve and your hydraulic tappets. This can usually be confirmed by the fact that the engine can be started and run again with no apparent problems after it has cooled down, whereas a sticking exhaust valve usually gives problems on the next cold start. What appears to have happened is that the engine has, some time in the past, been started with the throttle open and the resulting quick start may have been reported, but not the engine over speed. During this quick start, an inlet valve becomes stretched (Tuliped) and therefore slightly longer. If wear in the valve operating system has previously happened or the hydraulic tappet valve clearances have been set at the minimum value, the stretched valve expands lengthwise when the engine is running hotter at a high power setting and then does not quite seal on the cylinder head seat. This then allows the cylinder to run very inefficiently and in a worse case, the flame caused by the spark plug – through the slightly open inlet valve - starts combustion of inlet gases destined for other cylinders. One of cases known to us had the pilot report a loud “banging” noise immediately preceding what he described as ”someone putting a brake on the engine”. Again as in previous cases, the engine did not stop running – it simply had no power. The solution is to replace the Tuliped inlet valve and ALL the hydraulic tappets. It seems that the tappets “look OK” when the mechanic checks them, however the hammering during the engine over speed must do something also weaken their springs and they tend to fail, not doing the job of keeping a zero backlash in the valve train system. You can pick this by noisy (ticky) tappets when the engine is hot and sometimes a lack of “grunt” from the engine (the extra valve clearance means that the valve timing is out a little and the valve itself opens and closesby a corresponding reduced amount therefore restricting the amount of fuel/air mixture that can enter the cylinder.


E: TIP 4: The yellow arc on the tachometer

Is an avoid area of operation as a natural resonance frequency occurs which can cause the weighted stabiliser bar on the M/R mast to “hunt” damaging the Stab bar frame splines.


E. TIP 5: Cooling Fan Belt Failure

(a)  INDICATIONS:  There may be one or two bangs from behind the cabin.  Sometimes the fan belts will hit the cabin firewall with more noise or may hit the control tubes with some effect felt through the controls. In some accidents, it is suspected that the fan belts have damaged the fan which in turn has cut or damaged primary flight controls

Within seconds of the belts failing, the Cylinder Head and oil temps will start to rise.  The rise is rapid and the limits will be exceeded within a short time. The rate of temp rise is related to ambient temperatures, operating temps of the engine at the time of the initial failure and the subsequent amount of power (energy) delivered by the engine

Sometimes one belt will fail with all the above noises but if the other belt remains intact it will drive the fan and cool the engine.  On the other hand, if the broken belt gets caught in the fan and is spun by the good belt there will be a horrendous bi - frequency vibration.

(b)  ACTIONS: The Pilots reaction will depend primarily on available altitude. If there is somewhere to immediately land safely, enter a needles joined auto rotation and carry out a termination with power; land using power and shut down the engine. Depending on the cylinder temp, idle the engine for a short period of time to try and avoid damage to the exhaust valves and (if fitted) the turbocharger.

If there is no immediately landing site, the pilot has to decide how far to fly the helicopter to a suitable area.  The engine has been known to seize and the transmission internals slightly damaged by heat after 60 seconds of powered flight, but - if this means a safer landing area is reached, this is better than damaging the entire machine by throwing it into a bad area immediately the belts fail. Further flight after fan belt failure requires the pilot to have intimate knowledge of his machine's capabilities in powered and autorotative flight.

This is a pilots decision and can only be decided in each particular situation.

In the situation of one belt being caught in the fan the vibration may provoke the decision to auto rotate immediately.


E. Tip 6:  Engine Fires


This normally occurs after a backfire in a flooded engine.  The backfire will set the paper air filter on fire.  If the helicopter is fitted with a mixture control put it to I.C.O. and motor the engine to suck all fuel and fumes through the cylinders.  A fire extinguisher maybe required to be discharged into the intake if this is not successful.


Because of the engine location this may not be easy to detect.  If as fire is suspected land ASAP and use an extinguisher.


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