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 "FLAT PITCH MANIFOLD AIR PRESSURE"

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Piston engine helicopters have a unique warning system and safety device built into the helicopter that is often neglected and not understood by many pilots.

This device is none other than the humble Manifold Air Pressure Gauge - similar to the picture above.

The Manifold Air Pressure (MAP) gauge measures the amount of air passing into the cylinders from the carburetor. (At any fixed Engine RPM the MAP is also an indicator of Brake HP produced by the engine.)

The air flow is controlled by the pilot, through movement of the throttle and/or collective. These, in turn, move rods or cables which are connected to and operate a round disc in the carburetor called “the throttle butterfly valve”. 

Most Australian and NZ piston machines are sourced from the USA and the Aus MAP gauge is calibrated in “inches of mercury”.  A low power setting would be 10 inches while 28 inches is near to maximum available power in most normally aspirated engines @ sea level.

When the collective is fully down, an accurate and calibrated MAP gauge is actually indicating how much engine Horse Power is required to drive ALL the helicopter mechanical systems under a particular configuration & load and DA.

For example, @ S/level, a typical non – turbo charged piston engine will indicate 12 inches of MAP when the collective pitch control is fully down and the rotor is operating at flight RPM (104% on the R22).

Because the collective is full down, the MAP gauge is now indicating what is called “Flat Pitch MAP”.

Can we use this flat pitch information to help us with safety data about our metal steed? 

Flat pitch MAP easily indicates if your engine has:

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a sticking valve on startup

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poor sealing (worn) piston rings

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a fouled spark plug

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other mechanical defects

A machine in good mechanical condition, at sea level usually indicates between12 and 13 inches of MAP to maintains the main rotor RPM @104%.

If any problem is present, 12/13 inches will maintain less main rotor RPM (e.g. 99% instead of 104%)

If this were the case, you would now have to open the throttle butterfly valve further to allow more fuel/air mixture to restore the  main rotor RPM to 104%.  

This would be indicated as in increased MAP reading above the normal 12/13 MAP - (e.g. 13.5 or 14 inches MAP)

In this example, more Throttle results in an indicated increased MAP which in turn means that more power than normal is being used to maintain normal Rotor RPM and therefore something is faulty on the machine.!!!

Like the used car or TV salesman, the MAP gauge has more to show you if you want to look.

It can also warn you of many other problems apart from mechanical wear within the engine. 

Other common causes of higher than normal MAP are

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CARB HEAT - either positive or accidental selection of heat “on” will cause a power loss (more power is now required due the dense dense hot air)

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MAGNETOS - Check that both are “on” - selection of just one mag can happen, can’t it!

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TRANSMISSION Systems - Binding of a bearing or clutch, main or tail rotor gear box deterioration. Do not forget other mechanical systems such as cooling fan drives

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BLADE RIGGING - One I often find - main rotor blade low pitch stops are incorrectly set too high, causing extremely low rotor RPM in autorotation and high blade drag at flat pitch on the ground

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ERODED LEADING EDGES ON TAIL OR MAIN ROTOR BLADES - especially R22 Tail Rotor Blade tips

You now have many good reasons to monitor your MAP gauge, especially during the first start of the day run-up checks at max power-on Main Rotor RPM (R22 = 104%)   It can even indicate a trend towards a problem creeping up on you over a period of time if you see a small (half an inch) increase of MAP slowly becoming an extra 1 or 2 inches.

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