Using an R22B during a training flight yesterday (11- 9 - 00) the instructor noticed that RPM were 'lagging' when the lever was raised toward the (already established from the placard) maximum continuous power limit.
After taking control and making further investigation it was noticed that RRPM decayed quite rapidly when less than max. power was demanded. The low rotor horn didn't quite blow although it was not far off doing so. A less prudent demand for more power would no doubt have had it buzzing.
Wisely, the Instructor made the decision to return to base and treat the rest of the flight as being in limited power conditions. Re - checking power available calculations revealed that whilst OAT was not very high, (16C) pressure altitude and humidity were low.
The machine was well below its MAUW but RRPM decayed dangerously when full maximum continuous power was required. Magnetos checked out fine, the engine ran smoothly. Fuel sufficient for one hour remained in the tanks, having no contamination. Fuel flowed freely into the Carburettor. The Governor worked satisfactorily up to the last couple of inches MP when RRPM fell behind. Carb heat was only set to keep the needle just above the yellow band. Correlated flight (governor switched off) revealed the same problem, decaying RRPM near max. continuous power limit. It seemed reasonable to assume that this check ruled out any governor fault.
The aircraft was later inspected by its normal maintenance organisation (only 20 hrs flown since last 100 Hr check). The air filter was discovered to be WET through with water. An exhaust leak from a joint immediately underneath the hot air collector duct (for Carb. heat) positioned around the exhaust manifold was found to be directing exhaust solids to the air filter when heat was selected. Whilst this would not normally block an air filter so efficiently, in this case the particles were 'stuck' to it by the wetness, starving the engine of its air supply. The engineer described the air filter element as being "like concrete".
A new filter was fitted and the problem was solved immediately, no RRPM decay and full max. cont. power available with no further problems.
The owner/operator recalled that the Helicopter had been outside during a short but heavy rain shower the day before the occurrence and thought it likely that rain falling diagonally or horizontally from the Helicopter's right side had run down the air intake (placed on the right hand side of the fuselage). Further discussion led us to believe that even vertically falling rain could collect on top of the fuselage and then 'track' its way down the right hand side and down the intake.........
The stage was set. The two elements of this problem (wet air filter, exhaust solids) that may not cause any problem in isolation were drawn together in that all too familiar way - A combination that often results accidents.
That aside, an interesting point to note is that most pilots and instructors discussing the problem (myself included) were under the impression that the air filter box had an emergency bypass flap (alternate air source) designed to allow unfiltered into the Caburettor should the filter became blocked. It has become clear that this is NOT the case, the emergency flap was not fitted to this R22 Beta (1990).
It appears the alternate air source was deleted from production R22's about the same time as the R22 Mariner was unveiled. It is beleived that the reasoning behind the removal was to stop water being inducted when landing the Mariner on water, the loss being carried over to all other R22 models.
In any event, this particular R22B certainly does have an alternate air source (unless it has been repositioned and we can't find it) and it has been suggested it has not been fitted to R22 models for some time.
Anyway, the overall message in this was very clear to me - I thought that I had an emergency source of unfiltered air when I flew this Helicopter but I HAD NOT (and I had failed to notice that I hadn't!). In addition, the possible entry of water down the side mounted air intake needs to be carefully considered by all in the future - Alternate air supply flap fitted or not.
It also raises more questions for me;
(1) Although max. power available is calculated from OAT and pressure altitude before flight begins (which the instructor had done) it may only be discovered to be NOT actually available until a power check at 53 kt is done, maybe pre-landing ? - I'm considering moving that 'max power actually available power check' to the climb-out. It is the first chance you really get to do it (depending on the surroundings).
(2) Could the dampened air flowing through that air filter have the same effect as 'visible moisture'? What if the the pilot thinks there is no moisture going into the Carburettor because the air around the Helicopter is dry and clear, ambient conditions not conducive to carb. icing when the hard reality is that inducted air becomes damp enough to cause icing?
(3) Did the restriction of the air intake result in overreading of the MP gauge as actual manifold pressure would be lower than is usual? (A theory easily depicted by running a vacumn cleaner and then putting your hand over the nozzle, listen to the motor note before and after this action and just imagine what a vacumn gauge would read in both conditions if connected to the suction hose).
(4) How many accidents in the past may have been incorrectly attributed to 'Pilot Error', either through (a) poor appreciation of power available and subsequent overpitching or (b) poor attention to CAT and failure to apply Carb. heat, when the biggest contributing factor was a soaking wet air filter which had dried out by the time the Helicopter was inspected by AAIB?
Enough, but it is thought provoking eh?
The fact remains -
Risk of overpitching in this case (especially for a low-time pilot thinking themselves 'safe' in the knowledge that the Helicopter has maximum power available in favourable conditions, is well below its MAUW limit AND has a governor to manage RRPM) is just oh - so - very CLEAR !
I think we all need to think on this one, long and hard.
S. P. Sparrow