The Savvy Aviator #65: What's Your Fuel Flow at Takeoff?
Premature cylinder problems are epidemic. Hardly a day goes by that I don't hear or read about an aircraft owner having to pull one or several cylinders at annual due to poor compression with leakage past the exhaust valve. More often than not, the afflicted airplane is powered by a fuel-injected TCM engine. The cause of this epidemic seems to be the confluence of several contributing factors.
TCM seems to have had some manufacturing problems during the late 1990s and early 2000s that resulted in less-than-perfect valve-to-seat concentricity. (I think they've fixed this problem in current production, although it's hard to be sure yet.)
Also, maintenance shops and mechanics have been slow to adopt the guidance TCM issued in SB03-3 (click here for the PDF) urging mechanics not to pull cylinders due to low compression without performing a borescope inspection and identifying the cause of the low compression. Although it's been five years since TCM issued that service bulletin, I'd guess that more than half the shops that work on piston-powered GA aircraft still are not performing regular borescope inspections. As a result, we're still seeing a lot of cylinders pulled unnecessarily.
But I think one of the biggest factors contributing to early cylinder demise is inadequate fuel flow at takeoff. These engines require a very, very rich mixture to avoid excessive combustion temperatures and pressures at full takeoff power. If the mixture isn't rich enough, the cylinder assemblies will suffer ... particularly the exhaust valves.
How Much Fuel Flow Is Enough?
If you ask most pilots "how much fuel flow is enough at takeoff," most would make reference to the POH or the top-of-the-green on the fuel flow gauge. In fact, I've seen many pilots actually adjust the mixture control on takeoff to reduce fuel flow because the fuel flow needle was flirting with the red line.
This is not a good idea. Fuel flow at takeoff is like tire pressure: too much is better than too little. A little excess fuel flow on takeoff might reduce takeoff power by a couple of percent, but a little shortfall can overstress the engine and fry the exhaust valves in short order. I'd much rather see takeoff fuel flow a tad over red-line than significantly below it.
The "gold standard" for adjusting fuel flow on fuel-injected TCM engines is a 39-page service bulletin called SID97-3E. It's one that every owner who flies behind a fuel-injected TCM engine should be intimately familiar with. You can download your own personal copy of this important document from the TCM web site by clicking here (PDF).
If you look at the preamble of SID97-3E (see Figure 1), you'll see that TCM recommends adjusting the fuel system at initial engine installation, at every annual or 100-hour inspection, any time a fuel system component is replaced, and any time fuel flow seems to have drifted off-spec. Few shops actually do this routinely at annual inspection, but that's TCM's recommendation.
About half of SID97-3E's 39 pages are devoted to tables of fuel flow specifications for every model of fuel-injected TCM engine. For illustration purposes, I've extracted the specifications for the fuel-injected engines most commonly found in Beech Bonanzas and Barons (see Figure 2):
For example, if you fly a Bonanza with an IO-520-BA engine, the table calls for full-power fuel flow (standard day, sea level, 2700 RPM) to be 23.2 to 24.9 GPH, or equivalently 136 to 146 PPH. It also calls for unmetered fuel pressure at idle (600 RPM) to be 9.0 to 11.0 psi.
Read the Fine Print
Many A&Ps interpret this to mean that any fuel flow value within that range is okay, but in fact that's not quite right. If you read the fine print of SID97-3E, you'll find a couple of very important notes that mechanics often miss. Here's one:
In other words, the fuel system should be set up to produce 24.9 GPH or 146 PPH at takeoff, and anything less will compromise cooling and cylinder longevity. That typically translates to a fuel flow indication right at red line on the fuel flow gauge. (In my experience, it's not a bad idea to adjust the system 0.5 GPH or 6 PPH higher, just for a bit of extra cushion.)
Here's another important note that's often missed:
So not only not only is it important to adjust full-power fuel flow to the maximum limit, but it's also important to adjust idle-power fuel flow to the minimum limit (9.0 psi in the case of the IO-520-BA). Only by adjusting the system this way can you be sure of getting sufficient fuel flow at part-throttle settings.
Finally, there's the sticky issue of how to adjust fuel flow for engines that have been modified with aftermarket alterations like GAMIjectors and turbonormalizers. Here's what SID97-3E has to say on that subject:
In other words, TCM says "follow the STC-holder's instructions." Unfortunately, some STCs do not provide guidance for fuel system setup, which leaves the owner and his mechanic pretty much on their own.
A Quick Sanity Check
If you don't have your copy of SID97-3E handy, here's a quick rule-of-thumb you can use:
- For a normally aspirated fuel injected engine designed to run on 100-octane fuel (8.5-to-1 compression ratio), takeoff power fuel flow in GPH should be roughly 9% of the engine's maximum rated horsepower. (For example, an IO-520 rated at 285 horsepower should flow about 25 to 26 GPH.)
- For a factory-turbocharged engine (7.5-to-1 compression ratio), takeoff power fuel flow in GPH should be roughly 10.5% of the engine's maximum rated horsepower. (For example, a TSIO-520 rated 310 horsepower should flow about 32 to 33 GPH.)
This rule-of-thumb isn't as precise as looking it up in SID97-3E, but it'll at least make sure you're in the ballpark. Another side benefit is that the rule-of-thumb works for Lycomings and carbureted Continentals (assuming you know the compression ratio, which you can look up in your engine's Type Certificate Data Sheet by clicking here).
After reading this column, if you have even the slightest doubt about whether your takeoff fuel flow is adequate, go get it adjusted—and make sure you tell your mechanic that you'd like it to be on the high side. The life you save may be your own ... cylinders!
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