Automatic crossfeed is a popular topic of discussion at King Air Academy for several reasons. The first reason has to do with why we go to recurrent training in the first place – to review procedures like crossfeeding, which we rarely perform on normal flights. The second reason is to discuss the potential pitfalls when using the automatic crossfeed feature.
This article applies to the auto crossfeed operation in the A90, B90, C90, C90-1, C90A, C90B, C90GT, C90GTi and C90GTx King Airs.
A quick review: The PT6 engine incorporates a high-pressure, engine-driven fuel pump to provide high-pressure fuel to the fuel nozzles. Without this high-pressure pump the engine will not operate. To prevent cavitation, this high-pressure pump needs head pressure supplied by an electrically driven, low-pressure boost pump. Without head pressure, the high-pressure fuel pump will fail over time. Hence, the 10-hour operating limitation with the fuel press light illuminated. The boost pump cannot take over for the high-pressure fuel pump; it does not put out enough pressure. Therefore, we want to be nice to the high-pressure fuel pump. This is where the automatic crossfeed feature comes into play.
The designers of the 90 series models listed above decided to include an automatic crossfeed feature. If a boost pump fails, this feature would guarantee head pressure from the remaining boost pump to both high-pressure fuel pumps (one for each engine).
The King Airs listed only have one low-pressure boost pump per side to prevent cavitation of the high-pressure pump. If a boost pump fails, we can use the remaining boost pump for crossfeed operation to provide head pressure to both high-pressure fuel pumps.
The straight 90 and straight 100 King Airs have two low-pressure, electrically driven boost pumps per side. All other King Airs not listed have one low-pressure, electrically driven boost pump and one low-pressure, engine-driven boost pump. With two boost pumps per side, there is a standby boost pump ready to take over if the first one fails; no need to crossfeed.
In theory, the auto crossfeed sounds great – it will prevent damage to the high-pressure fuel pump if its boost pump has failed. However, it is important to understand when crossfeeding the fuel provided to both engines is coming from one wing only. The side with the operating boost pump provides fuel to both engines. We are now only using half of the fuel onboard. If we are not careful we could flame out both engines.
In 2017, the FAA published a Safety Alert for Operators (SAFO 18001) regarding the King Air 90 series auto crossfeed. A SAFO addresses a specific safety issue and contains important safety information and possible recommended actions to take. Why was SAFO 18001 produced? More than one operator has had a boost pump failure and did not notice the problem until both engines shut down.
One statement made by the SAFO 18001 is “… there is a clear risk that a simple boost pump failure, if not managed properly, could result in a dual engine failure.”
You may ask, how is it possible for a pilot to miss this obvious issue? It can be more subtle than you think. First, we need to describe the annunciators. In early King Air 90 models, only a red master warning flasher is installed. This light is located on the glareshield in front of the pilot’s face and will flash to get the pilot’s attention when any red annunciator illuminates. In this way the pilot will be alerted to a red annunciator light immediately. These early 90 models do not have a yellow master caution flasher. This makes it easy for a yellow annunciator to illuminate and not be noticed. Later 90 models do include a yellow master caution flasher.
The pilot operating handbook states that we should set the crossfeed to the auto position prior to takeoff and leave it there for the entire flight. If a boost pump fails, the crossfeed valve opens. Will we get an annunciator light? Hmmm, boost pump failed … we should get a red fuel press light with a red master warning flasher. The crossfeed will automatically open and the yellow crossfeed light will illuminate. As a result of the crossfeed valve opening, the red fuel press light extinguishes. The red flasher remains flashing with no red fuel press light.
This happens fast. The pilot may cancel the red flashing warning light and only see the yellow crossfeed light illuminated. The pilot may think, “Ahh, it’s just a yellow light, no big deal,” and not give it a second thought. Ignored, the likelihood of a dual-engine flame-out is high. Don’t be that guy – get the checklist out! The checklist will have the pilot move the crossfeed selector to closed. This will illuminate the fuel press light on the side of the failed boost pump. Now that the pilot understands that a boost pump has failed, proper crossfeeding of fuel can be managed for the rest of the flight.
An additional problem with the auto position selection of the crossfeed valve is the fact that the auto crossfeed can happen very fast. Due to the crossfeed valve opening so quickly when the boost pump fails, the red fuel press light may not illuminate at all. In turn, the red master warning flasher will not illuminate. In this situation, the yellow crossfeed light is the only light to illuminate.
The early King Air 90 models do not have a yellow master caution flasher; now the stage is set for the pilot not to notice the crossfeed light and burn fuel from only one wing unknowingly. A dual-engine flame-out scenario is primed and ready to go. Quoting from the SAFO 18001, “If that happens, options become limited … if crossfeed remains open, the HP fuel pump may instead only suction air through the empty crossfeed line.” Forget about trying an engine restart with the crossfeed open, sucking air; it will likely fail.
As you can see, it is very important to scan not only the annunciator panel but also the fuel gauges at certain intervals during flight.
The pilot who does not monitor the fuel until it is dangerously low on one side now has a problem. The fuel is high on the side of the boost pump failure and low on the side of the operating boost pump. The pilot has discovered the hard way that you cannot crossfeed from the side with a dead boost pump. The dead boost pump can’t push the fuel across to the opposite engine. The pilot is now in such a predicament that he will need to stop crossfeeding to prevent a dual-engine flame-out. The engine on the side with the operating boost pump will have no fuel remaining shortly and will have to be shut down. This leaves the pilot with one engine suction feeding but it is better than flaming out both engines.
SAFO 18001 recommended action:
- In the event of a boost pump failure, if the pilot chooses to continue flight with the crossfeed valve open, adequate fuel quantity should be verified on the side with the operating boost pump, considering fuel burn on that side will be double with the crossfeed open.
- In the event of a boost pump failure, if crossfeed remains open and fuel is depleted on the side with the operating boost pump, a dual-engine flame-out will most likely occur.
- In the event of a boost pump failure, the crossfeed valve must be closed for the HP fuel pump to scavenge-feed fuel from the side of the inoperative boost pump.
- In the event of a boost pump failure, proper fuel monitoring and management are crucial to avoid fuel starvation, leading to engine failure and/or fuel imbalance beyond limitations.
- Did the designers really need to include an auto crossfeed feature? In short, no. Without it, in normal operations, the crossfeed valve is closed. During a boost pump failure, the red fuel press light illuminates. This makes it clear which boost pump has failed, allowing us to crossfeed accordingly. We have 10 hours allowable for the high-pressure pump to suction feed during a boost pump failure. This is not a time-critical situation – no need to automatically open the crossfeed valve immediately. However, the Beechcraft checklist still calls for it to be in the auto position for the entire flight.
A competent, knowledgeable pilot should be aware of the status of the aircraft. The occasional scan of the engine instruments, fuel gauges and annunciators needs to be completed regularly. Following the Beechcraft procedures, with the crossfeed in the auto position for flight, when a crossfeed light illuminates, the pilot should recognize the light, get the boost pump failure checklist out and follow it. It is that simple. Understanding the pitfalls of the automatic crossfeed system, completing recurrent training and following checklists will reduce the level of risk.