MAP Gauge does not show "power"
Available pilot literature says that the MAP gauge reads out
power being used, and can therefore be used to determine how
much more "power can be pulled" (i.e. up collective.)
This is a common misconception that will be covered shortly.
The reason for this document is covered in the last section.
- Pilots of fixed-wing variable-pitch props: the parts that
talk about the gauge apply to you also, but the rest doesn't
because you guys can vary the engine RPM. Us heli
pilots have it around maximum RPM the whole time.
- There are no pictures here. The concepts are easy
to understand if you understand how an internal-
combustion engine works. (The Jeppeson Private Pilot Manual
is one resource that covers this very well. The FAA Rotorcraft
Flying Handbook is another.)
- I'm going to use short terms and acronyms where a conflict
in regional spelling or pronunciation might otherwise cause a
distraction (e.g. "carb").
- A simplified internal-combustion engine refresher
- The intake manifold venturi
- What do various reading ranges mean?
- What is MAP anyway?
- How do the controls affect it?
A simplified internal-combustion refresher
The engine is actually one big sucking machine. As the starter
engages, the pistons move, and in their intake phase they suck
air from the intake manifold. The throttle controls how much
air can go through.
It's not necessary to discuss the other parts of the engine as
they really don't relate to the MAP gauge here.
Just remember, the engine is one big air-sucking machine, and
the intake manifold is where the air comes from, and the
throttle butterfly-valve is what restricts that airflow.
The intake manifold venturi
Depending on design, either before the throttle butterfly
valve or after exists a venturi that takes the air from the
"outside" and expands the pipe diameter thereby decreasing
the ambient pressure (but of course not changing the volume).
The intake pressure is external ambient air pressure. This
is 29.92 in hg at 0ft and 15C. Ambient air pressure decreases
by "about" 1in hg per 1000ft but of course since the atmosphere
doesn't end at 29,920ft it's a nonlinear relationship. For
heli pilots flying at or below 10,000ft this is accurate to
within 0.5in hg (at 15C).
Map Gauge and Intake Pressure
The MAP gauge has a sensor that measures this air pressure.
This is why when the engine is not turning, the MAP gauge
displays the ambient air pressure. Yes, that's right, it's
a barometer. Yes, you can check it against your altimeter.
Checking Your Altimeter Against the MAP Gauge
- Using the altimeter adjustment knob, set the pressure to
29.92in hg (pressure altitude).
- Read the altitude in thousands of feet
- Subtract that from 29.92 (or 30 to make it simpler)
- That is what your MAP gauge should read.
If your MAP gauge reads as expected skip this section.
If your MAP gauge does not read as
30-(alt/1000) then one of the gauges is in error.
Get your altitude. Possible sources are field signs,
ATIS, sectional charts, your memory from the last time
you flew, etc.
GPS receivers can make this easy to figure out.
- Get your "exact" altitude. Note
that since we only care about 1,000ft units,
if you are using a GPS so long
as the GPS reading is within 500ft its reading is
"exact enough for this.
- Subtract your exact altitude in 1,000ft
units from 29.92.
- If your MAP gauge matches the number, your altimeter
- If your MAP gauge does not match the number, note by
how far it is off, and remember to make that correction
mentally every time you look at the MAP gauge until you
can get it fixed.
Past the venturi is an area of lower pressure. The
simplest reason for this is that this lower intake
manifold pressure provides a "suction source" which
allows the carb to identify how much air is being
sucked in, and feed fuel accordingly.
There is also another MAP sensor that reads this
pressure. The MAP gauge then displays the difference
between the two as a positive number.
The MAP gauge shows a pressure DIFFERENCE
The higher the number, that means there is a huge
pressure difference, meaning there is a lot less
than ambient pressure in the intake manifold, so
the throttle is either closed to some degree, or
the engine is working so hard it's sucking so much
air that not enough air can make it to satisfy
The lower the number, that means pressure after the
venturi is exactly the same as ambient pressure.
This will never occur at helicopter operating alt ;)
What do various reading ranges mean?
The throttle is NEVER fully closed. If it was, the
engine could not idle. However, the reason MAP
gauges don't show large numbers at idle is because
the engine isn't rotating quickly and there is no
power demand on it, so it is not sucking in
a whole lot of air, so there's not a big suction
past the venturi. Hence the pressure diff is
When seeing smaller numbers, the engine is not
When seeing larger numbers, the engine is not
getting the air it wants.
If the throttle could ever be fully closed, MAP
Gauge would show barometric pressure.
NOTE: It's clear that if all the MAP Gauge did
is show difference between two sensors, then
with engine off it should show zero. However,
when there is no difference, it just shows the
reading. When there is a difference it shows
suction (negative air pressure) as a positive
number in units of in hg.
What is MAP anyway?
Manifold Absolute Pressure
How do the controls affect it?
Before we get to that...
Why did you bother writing this document?
The current published literature makes mention of
the MAP Gauge only in terms of function. It also
indicates the controls in terms of function (e.g.
"more prop pitch increases MAP" and "higher throttle
increases the MAP". The other literature not
supplied to pilots and even Internet resources
(ask.yahoo.com, wikipedia, etc.) do not explain
the real relationship.
Further, since rotary-wing pilots don't vary RPM,
the only control is the blade pitch (collective)
and understanding how the same collective setting
can provide totally different MAP readings is
What do the controls do?
At idle, there is no demand for the engine, and
RPM will sit in the idle range. (75% on a Robinson
R22). The MAP will be somewhere around 11-15. The
throttle is not wide open, but the demand on engine
power is slight (keep the rotor turning) so the
amount of suction (air-starvation) is slight.
Remember, even full atmospheric pressure at sea
level (29.92in hg) is only 14.7 PSI, or half that
found in your car tire.
Once the helicopter is about to hover IGE the
increase in collective pitch causes a greater
resistance on the rotor, and more resistance
to the engine rotating, and more torque requirement,
and therefore to keep the RPM in the range,
the engine wants more air and more fuel and
the suction increases (MAP gets higher).
Once IGE hover is established, MAP is high
because of the inefficiency
of a rotor working in its own downwash.
Once forward flight is established, MAP
immediately reduces even though not
only is the same altitude being maintained,
but forward motion is ALSO being exerted,
(and consequently ALSO TR resistance increased
to compensate for MORE torque). You'd think
that with all this "requirement for more power"
MAP would go up. Nope, it goes down. Why?
The actual energy being exerted is less
because the rotor efficiency is much greater.
Now you start climbing, again MAP goes up. This is
because the demands of climb are more than
that of forward flight so the engine is
air-deprived. If you COULD give it more
throttle it would be happy (fixed-wing
guys CAN. We heli guys can't.) So we
stay within our operating parameters, and
maybe get some more forward speed to make
our rotor more efficient.
OGE Hover until settling (VRS). Lots of
"demand for throttle" but satisfied by
throttle so no high MAP.
Rules of Thumb
MAP is not "Power being used" nor "Power available"
- MAP is air deprivation at the intake manifold
- Too much air deprivation can be cured by opening
the throttle (we can't do that) OR by decreasing engine
- Attain ETL
- Get forward airspeed
- Trim and attitude
- Collective pitch reduction
I hope that was helpful. Any additions or questions,
drop me a line at the following in reverse:
net dot wetwork at gavron.
Initial Version: 2009-01-23