Updated: Jan 30
You will be introduced to Manifold Pressure - MP, when you convert to a Complex Aircraft, ie one with a Variable Pitch. Many variable pitch aircraft also have retractable undercarriage, but not all.
With a fixed pitch propeller you are used to having your RPM increase when you open the throttle, and decrease when you reduce or close the throttle. Your throttle is usually black, and your mixture is usually red, and you will have only those two engine - fuel & RPM - controls.
When you have a variable pitch propeller , you have THREE controls, a black throttle control, a blue pitch control and a red mixture control. The pitch knob controls the pitch angle of the propeller blade. When it is forward, against the dashboard, it is fully fine, as you wind it away from the dash it becomes coarser and coarser in pitch.
This is similar to paddling a canoe, when you start out, your strokes are smaller, but as you speed up, you make your strokes bigger so you can go faster. Same basic principle with propeller pitch. This lets you get the best blade angle for a really good climb, which would then slow you down in the cruise, so you can coarsen the pitch in the cruise and fly faster, allowing you to get the best performance possible in both the climb and the cruise.
NOW GET YOUR HEAD AROUND THIS - Your THROTTLE now controls the MANIFOLD PRESSURE - which is a new gauge that has appeared on your dash, along with the pitch control. Your PITCH now controls your RPM ! (Your throttle used to control your RPM)
This is a typical Manifold Pressure Gauge. It has some tricks up its sleeve! The humble Manifold Pressure Gauge can be used as a rudimentary ALTIMETER!
Most aircraft can fly continuously at 25 inches MP and 2500 RPM, and it is common practice, after takeoff, to reduce your throttle MP carefully to 25" and then coarsen your pitch slowly to 2500 RPM, and then adjust your fuel flow as required per the graphs in your Pilot Operating Handbook. Power Pitch Mixture after take-off. When planning to descend, its the other way round, Mixture (richen), Pitch (set as required, often coarsen), Power, (usually reduced for the descent.)
Back to the MP Gauge as a rudimentary Altimeter... First, why bother knowing this? Many years ago, a well known and respected pilot based in Pietermaritzburg fatally crashed into a hill at about 4000ft near Karkloof on his Approach into Pietermaritzburg. After a lengthy investigation it was determined that his static vent had iced up in the IFR conditions, causing his altimeter to read 7000ft when he had actually inadvertently descended to 4000ft. He thought he was at a safe altitude. This is what icing can do to your pressure instruments.
Standard Pressure is 29.92" (inches) or 14.7psi (pounds per square inch) or 1013.25hPa (hectopascals)
29.92" = 14.7psi = 1013.25hPa
When your aircraft is OFF, your MP Gauge will read the ambient pressure outside. When you start your engine, the MP will drop. At FULL THROTTLE your MP Gauge will once again show ambient pressure.
At Sea Level, your Gauge will read 30"MP. It will decrease by about 1"MP for every 1000ft climb in altitude, so at full throttle you should see the following readings at the following altitudes:
In the example above, had our pilot been flying a normally aspirated twin, and not a turbo charged one, he could have taken full power to confirm his altitude, and if his MP read more than 23", then he would have known he was lower than his 7000 ft safe altitude. At 4000 ft it would have read 26"MP.
1"MP = +/- 1000ft Altitude - this is an approximation, a "rule of thumb".
Here is the exact calculation:
29.92 inches = 1013.25 Hectopascals
Therefore 1 inch = 33.865 Hectopascals
( 1013.25 divided by 29.92 )
Now 1 Hectopascal = 26 feet
Therefore 33.865 Hectopascals / one inch = 880.45 feet
But on a dark and stormy night , 1000 is close enough... and quicker to calculate...
Engine OFF - MP = Ambient Static Pressure
Engine Start - MP drops
Full throttle - MP back up to Ambient Static Pressure
NOTE - THIS MAKESHIFT ALTIMETER WILL NOT WORK FOR PRESSURISED AIRCRAFT!!