Fuel pressure

Thinking back, it seems like a hundred years ago that my instructor and I were sprinting across some of the barest regions of Central Illinois on a dual night cross-country flight in an old straight-tail Cessna 172. He noticed my eyes regularly darting across the instrument panel to one of the analog fuel gauge needles that had developed a nervous tick. He didn’t mention anything until we were post-flighting the airplane that night back at Willard Airport (CMI) in Champaign, Illinois.

“You seemed pretty concerned about that fuel gauge,” he said.

My reply was simple. “I didn’t want us to run out of gas.”

“What would you have done next if that fuel gauge—if both fuel gauges—had failed during the trip?”

I confessed I had no idea.

The next morning in ground school, he taught us a lesson that would stick with me for the rest of my flying life. “Never trust these old fuel gauges,” he said. “Either open the cap and look into the tank, read the amount of fuel the line person added last, or push a graduated stick into the opening and measure the result” (This was all before computerized fuel measurement existed).

He also emphasized the importance of knowing how much fuel the aircraft burned in an average hour. I learned to feel comfy with 9 gph. With 38 usable gallons when the tanks were full, that meant I had 4.2 engine operating hours. Deduct for a reserve, and I began using 3.5 airborne hours. I relaxed quite a bit knowing that simple arithmetic.

Skip ahead a few decades. I was a newly type rated co-captain on a Cessna Citation 650, Cessna’s first swept-wing business jet. The 650 systems were sophisticated enough to carry me above FL400 for the first time. I learned to plan 450 knots on short flights, depending on the altitude of course. I also learned that when we were heavy, we’d burn about 2,300 pounds of fuel to reach cruise altitude. But while attending FlightSafety, I verified the numbers I heard with people who were much more experienced. More so than the Citation II I’d been flying before, the Garrett TFE731 engines burned fuel at an enormous rate at low altitudes trying to create 3,650 pounds of thrust each.

One Saturday morning, my co-captain on a three-leg trip happened to be the department boss of our tiny flight department. We flew one leg to Atlanta from Chicago Palwaukee (PWK—now Chicago Executive), dropped a few passengers, and continued to Cincinnati to drop two more before heading home. The weather was about as good as it gets, with only a chance of rain around our arrival time in Chicago.

I ran the fuel calculations in my head as we taxied to the FBO in Cincinnati/Northern Kentucky International (CVG) and decided adding a thousand pounds to the 2,500 to 2,600 pounds we landed with would be a good fit. On a VFR day, with no delays, I knew we’d land with about 1,500 pounds. I asked my boss to tell the line we’d like 1,000 pounds. That’s when things became interesting.

“We’ll be fine. We don’t need any fuel,” he told me. For a moment I thought this might be a test of my command talents.

“That will make fuel a bit tight to me,” I replied. “Let’s just tell them we’d still like 1,000 pounds.”

His tone this time was spoken as the boss. “We don’t need any more fuel. I want to get back for my son’s game. Keep the right engine running and I’ll see the passengers off. You can pick up the clearance for the trip back.” I realized this was no suggestion.

I’d filed for FL230 so we wouldn’t spend too much time climbing before we could let our racehorse of a bird run free. He nodded and we were soon climbing out of CVG. The initial call to Indy Center wasn’t what I expected.

“Citation 250CM, stop your climb at FL180. That will be your final for today.” Then, as we were just northeast of Indianapolis we heard, “Citation 250CM, cross 10 southeast of Boiler at and maintain 12,000.” Great. I could almost hear that sucking sound those Garretts were making as the fuel flow increased. My co-captain read me the latest PWK weather. It was IFR with a 700-foot ceiling and a mile-and-a-half visibility.

By the time it was our turn for the 16 ILS I caught myself glancing at the fuel gauges. We had about 1,000 pounds remaining. The Chicago controller cleared us about two miles outside the final approach fix and mentioned PWK weather had dropped to 200 and a half. I focused on the job ahead and shot “a great ILS to minimums,” according to my boss. Just before I pulled both engines into cutoff, I saw our final fuel on board reading was 600 pounds.

I must have been shaking my head as I wrote down the numbers because my boss said, “That flight didn’t scare you, did it?”

“I should have taken that extra fuel before we left Cincinnati,” I replied. “If we’d missed that first ILS, our only alternative would be to run for 14 Right at O’Hare on emergency fuel.”

“But we didn’t miss, did we. So why are you even concerned?”

I shook my head again and finished the shutdown checklist. Every student I worked with in the 25 years since that day has known roughly how much fuel the airplane had on board before departure, and also learned never to be in too big a hurry to add more if they believed they really needed it.

Rob Mark is an aviation journalist and the publisher of JetWhine.com.