SR20 constant fuel flow? (technical)

It’s a beautiful day in San Francisco. I’ll bet I could see the Sierras on climb out, but all my club’s planes are taken, and I’ve got more than a year to wait for my own. Meanwhile, I puzzle over nerdy questions like this:

The SR22 cruise performance numbers show the fuel flow falling off as the temperature increases. (Actually I’m looking at Paul Traina’s spreadsheet – thanks Paul.) But the SR20 POH shows the fuel flow remaining constant as the temperature increases. Here are comparable excerpts from both:

SR22: 8000ft pressure altitude, 2500rpm, 21.7map:

ISA-30: 79%pwr, 19.1gph, 178ktas

ISA-00: 75%pwr, 18.4gph, 181ktas

ISA+30: 71%pwr, 17.9gph, 184ktas

SR20: 8000ft pressure altitude, 2500rpm, 22.2map:

ISA-30: 73%pwr, 10.1gph, 152ktas

ISA-00: 69%pwr, 10.1gph, 154ktas

ISA+30: 66%pwr, 10.1gph, 156ktas

The 22’s behavior makes sense to me. As you hold everything else constant but increase the air temperature, the air density decreases. So the air mass that both engines are pumping decreases. The 22 appears to be keeping its fuel/oxygen mixture constant, while the 20 is keeping its fuel flow constant (and thus making the mixture progressively richer).

Is this the “altitude compensation” that’s vaguely referenced in the SR20 manuals? And does the SR22 lack that device? What are the benefits? Do you spend more time fiddling with the mixture control on one plane than the other?

Curiously, the SR20 figures show this constant fuel flow phenomenon at all altitudes and temperatures except at 6000 feet. That table shows the same GPH numbers for ISA-30 and ISA, but drastically lower numbers for ISA+30. This only happens at 6000 feet – at 4000 and 8000 (and all other altitudes) the flow is constant. POH misprint? Or is everything else a misprint?

Of course, the correct Zen answer is to stop rationalizing and fly…

The IO550 in the SR22 does not have an altitude compensating fuel pump.

The ACFP simply allows you to leave things “optimally rich” at all levels of cruise. Most people I’ve talked to like it when it works, but wouldn’t feel bad about losing it.

I don’t miss it.


The IO550 in the SR22 does not have an altitude compensating fuel pump.

The ACFP simply allows you to leave things “optimally rich” at all levels of cruise.

It’s actually meant for the climb and descent, but doesn’t hurt anything in cruise (other than being too rich and throwing fuel away.) Just stuff it all the way forward on the way up and on the way down, and lean in cruise.

It’s particularly nice for those of us at high-altitude airports, as you don’t have to remember to lean on the ground (and don’t die if you forget.) The fuel flow-vs.altitude schedule on the wee placard in the SR22 lets you be the altitude-compensating gizmo. (In Beech products the fuel flow gauge has altitude marks on it.)

For what it’s worth, they took the altitude comp out of the 550s in Barons starting in 1997. There’s an SB to remove them in previous model years. Guess they didn’t work so hot in the 550.