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6 Answers

Manifold Pressure Gauge Requirement

Asked by: 10993 views Aircraft Systems

I have only seen  manifold pressure gauges in piston airplanes with constant speed propellers.

Question: Are manifold pressure gauges only found in piston airplanes with a constant speed propeller? 

 

Thank you for the feedback.

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6 Answers



  1. Kris Kortokrax on May 19, 2014

    No. I fly a 1928 WACO Taperwing with a Hamilton Standard ground adjustable propeller and a1942 WACO UPF-7 biplane with a Curtiss Reed fixed pitch propeller that have a manifold pressure gauge.

    The Piper Tri-Pacer in which I learned to fly had a fixed pitch Sensenich prop and a manifold pressure gauge.

    Just because a piece of equipment is not required, doesn’t mean it can’t be installed.

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  2. John D Collins on May 19, 2014

    In an aircraft with a fixed pitch propeller, RPM is the primary indication of power and a manifold pressure gage would not add any information relating to how much power is being developed by the engine.What affects power is the RPM, OAT, and altitude.

    On a constant speed propeller, the primary indication of power is from the manifold pressure as the RPM remains constant as long as the governor is able to maintain the selected RPM in the governing range between the high and low pitch stops. When the constant speed prop hits the high or low pitch stop, effectively it is a constant pitch propeller.

    Power is a proportional to air flow delivered to the engine when the engine is operated on the rich side of peak EGT. Unfortunately, we don’t have an indicator that indicates how much air is being delivered to the engine or it could be calibrated in units of power. Instead we use tables that provide all the parameters that affect the amount of air to the engine including RPM, manifold pressure, OAT, and altitude and lookup the answer.

    On the lean side of peak EGT, power is proportional to fuel flow. Many aircraft have a fuel flow indicator which is directly related to power when operating on the lean side. This makes determining power easy when operating lean of peak EGT, set a known fuel flow and you have a known power. On my Bonanza, if I set 12.5 GPH and I am operating lean of peak EGT, I am developing 65% power.

    So, yes a manifold pressure gage is only needed on an engine with a constant speed prop.

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  3. Kris Kortokrax on May 19, 2014

    91.205 states that a manifold pressure gauge is required if the aircraft has an altitude engine. It mentions nothing about a constant speed prop.

    Part 1 defines an altitude engine as:
    “Altitude engine means a reciprocating aircraft engine having a rated takeoff power that is producible from sea level to an established higher altitude.”

    This would seem to imply that an altitude engine is either supercharged or turbocharged.

    23.1305 (b)(5) requires:
    “A manifold pressure indicator for each altitude engine and for each engine with a controllable propeller.”

    The 450 Stearman I flew had a 2 position controllable propeller that isn’t the same as a constant speed prop, but it would still need the manifold pressure gauge.

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  4. Dan Chitty on May 20, 2014

    Thank you John and Kris. The feedback each of you have provided is very helpful.

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  5. snowpilot on May 20, 2014

    A follow up question….

    Is a constant speed prop system considered an altitude engine?

    Altitude engine is defined “a reciprocating aircraft engine having a rated takeoff power that is producible from sea level to an established higher altitude.”

    I guess I’m foggy on the definition of ‘power’

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  6. Kris Kortokrax on May 20, 2014

    No, a prop is not considered to be an engine.

    The definition of altitude engine implies that if an engine is capable of producing, say 200 horsepower at sea level, that it will be capable of producing that same 200 horsepower at some higher altitude, say 5,000 feet.

    Without some means of maintaining sea level air pressure in the intake (supercharging or turbocharging), the power output of the engine will diminish as you climb.

    This is a bit of an oversimplification, but you get the idea.

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