Aviation Fuel Crossfeed… Why Can’t We Do It?!

Every Pilot’s Operating Handbook (POH) for the various King Air models contains a prohibition against using Aviation Fuel crossfeed except when one engine is inoperative. In the case of the A90, B90, and C90 variants, the POH also allows crossfeed when one side’s boost pump is inoperative.
This prohibition means that we are not allowed to crossfeed (1) to correct a fueling imbalance, side-to-side, or (2) to consume the remaining fuel as rapidly as possible from a side that has sprung a large leak. Why can’t we do this? Why are we prevented from taking advantage of this desirable option in these two cases?
The crossfeed prohibition did not exist until after the POHs got revised in the mid- and late-1970s. Prior to that, pilots could and did use crossfeed as needed to correct a fueling imbalance. The case of a major tank leak is so rare – and probably only of great concern when far from land on an oceanic ferry flight – that I will not address it again in this discussion.
In fact, all King Air models, including the latest 350 variants, are designed and manufactured with sufficient pump capacity and fuel line capacity that feeding both engines from one side’s fuel supply can physically be done, even during periods of maximum fuel flow. Knowing this, it is apparent that the crossfeed limitation did not originate with the engineering designers; instead it came from Beech’s legal team.
In June 1974, a Beechcraft BE95 Travel Air – the 180 horsepower Lycoming-powered predecessor to the Baron – experienced fuel starvation to both engines, crashed, caught fire, and killed the four occupants. Investigation revealed that both engines had been drawing fuel from the same tank on one side, the exhaustion of which led to the double engine failure. The fuel selectors were positioned properly for this crossfeed situation. The pilot never repositioned the fuel selectors such that he could feed the engines from the existing fuel in the other tanks.
Although most aviation-savvy people thought this was an obvious case of pilot error, a jury trial found Beech to be liable for a “Failure to Warn.” Huh? I guess the jury thought there should be something like, “Thou shalt not feed all engines from one tank, run said tank dry, and not utilize the remaining fuel in other tanks!”
That tongue-in-check type of warning never got inserted into revised POHs, but the crossfeed prohibition we have lived with ever since came as a direct response to this infamous legal case.
A chapter in my “The King Air Book” discusses the C90’s fuel system in depth and one of the discussion points therein tries to highlight the absurdity of prohibiting crossfeed except with an engine out, while at the same time allowing the feeding of both engine’s from one fuel supply following a boost pump failure. In other words, the fuel system designers specifically intended feeding both engine’s from one supply while the lawyers, later, told them not to do so.
The fact is that any King Air’s fuel system can supply fuel to both engines from one side’s nacelle tank. It is quite obvious that the degree of risk has gone higher when this is done, since fuel contamination or exhaustion (of that tank) will affect both, not just one, powerplant. Realizing that increased risk, I am sure that few, if any. pilots would choose to feed both engines from one tank while near the ground: During takeoff, initial climb, approach, and landing. On the other hand, I believe most pilots would be willing to accept the increased risk while established in cruise at an altitude high enough to permit glide and airstart procedures in the event the unlikely happened.
The actual fuel imbalance limitation varies widely among the various King Air models, from no limitation being stated to a high of 1,000 pounds in the 200-series and to a low of 200 pounds in the C90B and later 90 models. Unless the fuel load can be equalized between the two sides, it means that our endurance and range calculations must always be based on the side with the lesser amount of fuel.
For example, if the FBO’s fuel truck broke down, leaving our B200 with about 1,300 pounds on one side (full main), but only 700 pounds on the other, instead of basing our fuel calculations on the total 2,000 pounds the airplane contains, we’d need to plan our flight based on an available fuel load of 1,400 pounds, double the low side figure. This assumes the two engine’s fuel consumption rates are the same, which is a reasonable assumption.
On the other hand, suppose we departed with the fuel imbalance – that a little aileron trim would easily handle – and by the time we set up cruise at top-of-climb we are now down to 500 pounds on the low side and 1,100 pounds on the high side. If we now use crossfeed to supply the high side fuel to both engines, in about an hour 600 pounds will have been consumed, the fuel would be balanced; we could stop crossfeeding and proceed with the original 2,000 pounds usable.
Do you see why it’s quite desirable to have that crossfeed arrow in your quiver of alternatives?!
If you are ever going to do this – of course, in direct violation of the POH limitation – it would surely be a bummer to forget that you were now feeding both engines from one tank, run the tank empty, and perhaps have a replay of the 1974 accident! Setting a timer, sticking a post-it note on the instrument panel, moving your watch to the other wrist … any “bugging” technique such as one of these is a great idea to remind you to go back to normal operation when balance is achieved.
Let me conclude by giving a brief tutorial about sending fuel from one nacelle tank to the opposite engine, or to both engines if you do not have an engine shut down. Although the fuel always originates from the outlet of the nacelle tank, realize that how the wing fuel gets into the nacelle varies quite a bit between the various models. For models with a filler cap near the wingtip, the nacelle tank is simply the lowest member of the set of fuel tanks that comprise the main system. For the LJ-series, the nacelle tank must be fed via the transfer pump or gravity flow, and gravity flow causes 28 gallons on that side to become unusable.
Because of a checkvalve that prevents flow into a nacelle tank from the crossfeed line, also realize that we never flow fuel from the tanks on one side into the tanks of the opposite side. No, we only feed engines with fuel in the crossfeed line, we never flow that fuel into tanks.
So, to crossfeed, first we need a shove coming from the bottom of a nacelle tank, and that push can only be supplied by an electric boost pump or standby pump. Second, we need a path to the other side: An open crossfeed valve and line. Third, we need to guarantee that an opposing shove is not coming from the other side.
For the LJ-serial number series, just move the crossfeed switch to Open and turn off the boost pump on the low fuel side. For E90s, A100s, and B100s, turn on the high side’s standby pump, move the crossfeed switch to Open, and make certain the low side’s standby pump is off. For the F90-, 200-, and 300-series, merely move the Crossfeed switch left or right, towards the lower side, and verify that both standby pump switches are off.
Here is something of significant importance: Since, with the exception of the 350 model, the crossfeed annunciator only indicates that power is being sent to the valve, not that the crossfeed valve actually opened, we could have a failure of the valve, leading to no crossfeed taking place, yet the annunciator could still illuminate. Similarly, if the feeding standby pump were to stop operation, crossfeed would cease but fuel flow would continue normally, each side feeding its own engine.
Therefore, there is only one 100% accurate verification that you are indeed feeding the high side fuel to the opposite engine or to both engines: The high side fuel quantity goes down and the low side fuel quantity remains constant. Make absolutely certain that you check the fuel gauges regularly and confirm carefully that indeed the high side is decreasing.
Twice in my 42-years of King Air experience, I have discovered airplanes in which crossfeed worked backwards! One case, a 200 had the crossfeed switch mis-wired such that it turned on the incorrect, opposite, standby pump. The other situation, a 350 had the left fuel gauges connected to the right side tanks and vice versa!
In closing, please realize that this article is instructional in nature, meant to increase the reader’s systems knowledge. Be careful out there!
If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at kblonigen@cox.net.