Do You Know Where Your Fuel Flow Is Tonight?

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Pssssst! Wanna know a secret that will help to keep your fuel-injected Continental engine running strong? Here it is: be certain that your fuel flow needle is fully at red-line when you apply full takeoff power.

The most important adjustment your mechanic can make on your fuel-injected Continental powerplant is to set the fuel flows properly. There are four separate adjustments, two at the fuel pump and two at the fuel control unit. Perhaps the most important adjustment, known as high unmetered fuel pressure, determines the fuel flow at takeoff power.

The "Limitations" chapter of your Pilots Operating Handbook lists the proper RPM and fuel flow required for full takeoff power. In most cases, these values correspond to the red radial markings on the tachometer and fuel flow gauges. (In some aircraft, proper takeoff fuel flow is indicated by a white arc on the fuel flow gauge.)

If your fuel flow needle isn't reaching (or even slightly above) red-line during a full-power takeoff, then your engine is operating leaner than it should be. Your are not achieving full rated horsepower and takeoff performance. The engine is not receiving adequate cooling precisely when it needs it the most. Your cylinder heads and exhaust valves are running hotter than is healthy for them. At worst, your engine might suffer from detonation damage.

At the CPA Tech Center, we always perform a full-power runup, checking for full rated manifold pressure, RPM, and fuel flow. We also check tachometer calibration with an electronic tach checker. Frequently, we find that these critical adjustments have been neglected during past maintenance.

A Lucky T210 Owner

Last week, for example, an owner brought in a T210N for an annual inspection. The engine was a TCM factory reman with only 313 hours on it. During the runup test, we discovered that the engine wouldn't achieve full rated takeoff power. Manifold pressure was only 34 inches (it should have been 36.5), RPM was only 2600 (it should have been 2700), and fuel flow only came up to 160 pounds per hour (it should have been 190 pph). The owner had wisely implemented an oil analysis program, and the last two reports had shown higher-than-normal iron content.

At the beginning of the annual inspection, we sent another oil sample to the lab. (The CPA Tech Center uses Engine Oil Analysis in Tulsa.) The lab phoned to say that iron, tin and copper were all up by a factor of six! We pulled a valve cover and removed the intake and exhaust rocker arms and rocker shafts. The rocker arm bushings were worn and had transferred metal to the rocker shafts. This is classic evidence of excessive cylinder head temperatures, causing oil breakdown and lubrication failure.

Apparently, whoever had installed the new engine in this aircraft neglected to check or adjust the fuel pressures before turning over the keys to the owner. In this case, the owner was lucky...we caught the problem before he experienced a catastrophic engine failure on takeoff.

It's All in the Book

Both Cessna and TCM go to great lengths to instruct mechanics on how to make these fuel pressure adjustments properly. All Cessna service manuals include instructions on setting low and high unmetered fuel pressure, idle RPM, and idle mixture adjustments. TCM also has a very complete service letter (M84-6) on this subject.

It is absolutely imperative that these adjustments be made whenever a new, rebuilt, or overhauled engine is installed. They should be rechecked at every annual inspection and readjusted as required. Unfortunately, this isn't always done.

Pilots should keep a sharp eye on the engine instruments during every takeoff run to ensure that the engine achieves full red-line fuel flow. It's better for fuel flow to be slightly over red-line than to fall short. Failure to achieve proper fuel flow is grounds for aborting the takeoff. And without fail, take the aircraft to a knowledgeable Cessna mechanic and have the fuel pressures adjusted before further flight.