Checking for proper takeoff power

troopernflight

troopernflight

Well-Known Member
What is the best way to tell if the engine is not producing normal power on the takeoff roll in a piston powered airplane? The RPM indications are always going to vary depending on the density altitude, correct?
 
Density altitude will cause a variation, but it should not be too noticeable if you do most of your flying in the same general area. The main reason RPM is restricted when takeoff power is applied is due to the load placed on the propeller from churning in stationary air. Check what the tach reads next time you go flying and use that as a benchmark. Flying 160HP Cherokees in FL I usually see 2350-2400 at the beginning of the takeoff roll regardless of the time of year with density altitudes ranging from SL to 1700 feet or so. Make sure you pay attention to the sound of the engine and engine instrumentation as well.
 
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The engine is making proper power if its static RPM (full power, not moving) is meeting the book numbers. As Rocketman said, it should not change very substantially due to changes in density altitude. Just from the experience of working with a lot of mechanics everyday, I believe the first thing that is usually checked when an engine isn't making the book numbers is a testing of the tach to make sure it hasn't developed some error before any major engine work is actually undertaken.
 
I'd recommend doing an initial glance at the RPM/Oil pressure/MAP etc to make sure everything is in the green (power check), and then do a "speed check" on your takeoff roll to make sure you're getting the expected performance. The rule of thumb is you should get 70% of Vr speed at half the takeoff ground roll. So if you expect to need 1000 feet for takeoff and Vr is 55 knots, you should get about 38 knots at 500 feet into the takeoff ground roll. If not, something is wrong.

You should constantly be evaluating the performance of the plane and engine right up till the abort point, which will be just before the midpoint down the runway for most airports with relatively long runways, to decide whether you need to reject the takeoff or not.
 
Muff3n said:
The engine is making proper power if its static RPM (full power, not moving) is meeting the book numbers. As Rocketman said, it should not change very substantially due to changes in density altitude. Just from the experience of working with a lot of mechanics everyday, I believe the first thing that is usually checked when an engine isn't making the book numbers is a testing of the tach to make sure it hasn't developed some error before any major engine work is actually undertaken.
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Mechanical tachs are notoriously inaccurate, and we always use an optical tach checker, even nothing is suspected to be wrong. I never trust the ship tachometer when doing run-ups/inspections.
 
When I was flying freight in. C210 I would do as the others have said and watch the RPM overall and then also double check that 70% of my speed was obtained for takeoff by half way down the runway.

Just my $0.02.
 
A naturally aspirated piston engine with a constant speed prop will not be able to produce more MP than what's available for ambient (you may get an inch or so from ram air, but that's negligable) .

The easiest way to check is to note what MP is showing on the gauge before engine start. When the engine is not running the MP gauge will indicate ambient pressure. At WOT during the takeoff roll, MP should be very close to what you observed prior to engine start.

Things like engine vibration, back-fires, mis-fires, non-linear RPM increase with throttle increase are some of the more obvious indicators something is wrong. These will be your main indicators for a fixed pitch. The maintenance manual should give you a static RPM indication for the type of prop installed. This is only as reliable as the tach. Regardless of how accurate the tach is consistency is what you're looking for.
 
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A normally aspirated fixed pitch powerplant loses ~3.0%HP every 1000 feet.
A N/A constant speed powerplant loses ~2.85%HP every 1000 feet.
 
The aircraft's TCDS will list maximum permissible RPM.

When someone squawks "low power" to me , the first thing I check is compression, spark plugs and intake/ exhaust leaks.

A student pilot may not recognize it, but even some one with a little but of time will realize the plane isn't making power during the T/O. Unless they've switched from a C172 w/ 180hp to a 150hp, low power is noticed immediately.
 
Thanks for the info everyone. I just got a little on edge the other day when it felt like I wasn't getting up to speed as quick as usual. It was 90 degrees, so I'm sure that density altitude had a lot to do with it. But it was the first time I actually looked at the tach while on roll, and I noticed it was only like 2300rpm. I guess that is a result of loading on the props, but I just wanted to make sure it was normal.
 
I'd recommend adding a Tach/MP and oil pressure scan to your takeoff roll every time. It gives you something good to do on the first few seconds of the roll before the ASI comes to life anyway.

When you get used to adding it to your takeoff scan you'll come to know what normal power is. It will vary with powerplant and prop pitch (cruise vs climb props on fixed pitch) and DA will affect it as well. Remember at a high DA you may lean for takeoff, the POH should provide some guidance.
 
With my experience in a non-turboed 206, you're going to hear the problem and then glance to verify the problem when something happens. We do have our takeoff scan to verify everything is "in the green" but I've had aircraft have issues without much change in the gauges from normal. If something doesn't feel or sound right, it usually isn't and you'll be more likely to hear or feel something before you see it in your gauges.
 
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