Aftermath: How Much Fuel, Really?

Some people just guess.

Aftermath Artwork

Aftermath Artwork

The King Air had been airborne for just 30 minutes, en route from Pine Bluff to Bentonville in Arkansas, when it began its descent from 16,500 feet. The weather was clear, and the 3,400-hour private pilot, 72, was on VFR flight following with Razorback Approach at Fort Smith.

Nine minutes after leaving 16.5, he told the controller that he wanted to change his destination to Fayetteville.

"Any reason for the change?" the controller inquired.

"Sir, fuel seems to be a little low, ah, could you give me vectors real quick please?"

"King Air 9JG, fly heading 200, vector for the right downwind entry Runway 34 at Drake, wind 310 at eight, the altimeter 29.85."

"Got it, 9JG."

Less than 20 seconds later, the pilot asked, "How far to Drake is it?"

"I'm showing Drake Airport from your present position niner miles."

"Niner miles. Is there anything closer?"

"King Air 9JG, Springdale Airport's at your, ah, at your 12 o'clock and 4 miles."

"[We'll] go to Springdale, 9JG, yes sir, I think I have it in sight. I am familiar."

The pilot contacted Springdale tower and was cleared to land on Runway 36. Half a minute later, he told the tower he would not make the airport.

Several witnesses observed the King Air maneuvering to land in an open field about 3 miles east of Springdale. It passed over 40-foot-high power lines. A wing dropped, and the airplane plunged to the ground almost vertically. Both occupants lost their lives.

There was no post-crash fire because there was no fuel in the airplane. Microscopic examination of bulb filaments revealed that annunciators on both engines had indicated loss of fuel pressure. The tanks were dry. Auto-feather was not armed, and the props had not been feathered.

The field for which the pilot was aiming was about 1,400 feet long and abutted other open fields. There were scattered trees, fences and other obstacles in the area, but with luck the landing might have been survivable for the people, if not for the airplane. It failed, in the view of the National Transportation Safety Board, because the pilot suddenly caught sight of the power lines in front of him, instinctively pulled up and, in doing so, stalled the right wing.

The King Air was equipped with two independent fuel monitoring systems. On a console to the left of the pilot was the fuel management panel. In addition to switches and circuit breakers, there were two gauges, left and right; each could be selected to display total fuel or nacelle tank fuel only, and was marked with a yellow arc indicating the minimum fuel required for takeoff, 41 gallons on each side.

The second system was a totalizer, which displayed, among other things, fuel used and fuel remaining. A totalizer does not measure fuel quantity; it tracks fuel use and subtracts the fuel burned from the amount in the tanks, which must be updated by the pilot each time fuel is added.

The NTSB tried to reconstruct the fuel burn on flights preceding the accident flight, but it was difficult. The pilot was reported to have been in the habit of adding only as much fuel as he anticipated needing for a flight, and many of his flights, according to a log retrieved by FlightAware (which records only flights that interact with ATC), were an hour or less. The most recent known instance of his topping off was for an 800 nm flight to Cozumel, off the east coast of Yucatan, almost a month before the accident. He had stopped at New Orleans on the way back, but how much fuel he got at New Orleans, if any, or at Cozumel is unknown.

After returning from Cozumel, the pilot had the FBO at his home field, Harrell, in Camden, Arkansas, add 25 gallons to each wing tank. He then flew 65 nm to Pine Bluff. That flight would have used about 210 pounds of fuel, or 31 gallons. Some avionics repair work was done at Pine Bluff, and the airplane remained in the shop's hangar until the day of the accident. The pilot did not add fuel before departing.

The distance from Pine Bluff to Bentonville is 172 miles, and flying it would use around 460 pounds of fuel, or 70 gallons. Since the airplane ran out of fuel while descending a little short of Bentonville, it would have used, say, 64 gallons since Pine Bluff, or 95 in all since leaving Camden. It must, therefore, have had only 45 gallons of fuel aboard before the 50 gallons were added there. Since it's only 250 nm, or roughly 100 gallons, from New Orleans to Camden, the pilot cannot have topped off at New Orleans; he must have left there with no more than 150 gallons aboard. The fuel quantity indicators must have been well down in the yellow when the airplane took off from Pine Bluff.

Investigators removed the totalizer from the wreckage. It powered up normally and displayed 260 gallons used and 123 remaining. Those numbers add up to the C90's fuel capacity and imply that the totalizer had last been reset — perhaps inadvertently — to "full," but the amount used was not consistent with the recent trips the airplane had made and the balance remaining bore no resemblance to what was actually in the airplane.

Various hypotheses could be developed to account for the discrepancy, but the NTSB settled on the simplest one: that the pilot relied on the totalizer rather than the fuel gauges to determine his fuel state, and evidently some error had been introduced into the totalizer.

The pilot's rather sudden discovery that fuel was "a little low" does support the idea that he had ignored the fuel quantity indicators. It was probably the low fuel pressure annunciator light that alerted him. But he must have believed the gauges, because otherwise, relying on the totalizer, he would have reported an engine problem rather than a lack of fuel to ATC.

The pilot's habit of adding small amounts of fuel before short flights, without apparently topping the airplane off unless he had to, created frequent opportunities for error in programming the totalizer. Fuel flow transducers are not perfectly accurate, but they are normally good to within a fraction of 1 percent. There is no plausible way that the accumulated error could reach 123 gallons.

On an airplane like the King Air, which has multiple fuel tanks, the only practical way to zero out totalizer drift is to fill the tanks. The fuel gauges, however, give a reasonably good indication of the fuel aboard, and a pilot should always consult both the gauges and the totalizer to verify that they agree at least roughly with one another.

Presumably, the rationale for carrying no more fuel than you need for the current flight is the performance penalty of carrying excess fuel around with you — "tankering" fuel. But the penalty for the extra weight is slight: Tankering 1,000 pounds of excess fuel at 220 ktas in a C90 costs less than 20 pounds an hour.

Pilots use different strategies for ensuring that they will not run out of fuel. Some always top off; some measure fuel with dipsticks; some don't trust fuel gauges and plan in terms of time and fuel burn instead. Estimating fuel use by hours flown is somewhat unreliable unless you pay close attention to power setting and, particularly, to mixture. Pilots who lean by feel may find that they use more fuel at night, because they are less accepting of slight roughness then. Carburetor heat increases fuel consumption. That is why a totalizer is such a useful thing; it eliminates many uncertainties.

The fundamental requirement is consistency among multiple sources of information. Gauges, totalizers and mental calculations of time in flight and power setting should agree; if they don't, something is wrong — and can eventually go very wrong indeed.

This article is based on the NTSB's report of the accident and is intended to bring the issues raised to our readers' attention. It is not intended to judge or to reach any definitive conclusions about the ability or capacity of any person, living or dead, or any aircraft or accessory.

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