Learning to know a new plane from a known family can be tricky. While studying the DA42 Aircraft Flight Manual (AFM, a.k.a. POH for US pilots) I see many similarities with the DA40 and it can be tempting to simply overfly some chapters or paragraphs. A careful study is important: the theory examination will come soon, my instructor will not tolerate any approximate answer, and it could ultimately be a life saver… For todays post, I prepared a summary of the fuel system.

The DA42 is equipped with two Thielert TDI engines. The fuel systems of TDI engines is slighty more complicated that those of AVGAS engines because a significant part of the fuel is not burnt and sent back to the tanks. This helps to control the fuel temperature, but pilots must be careful about the quantities and know were the extra-fuel is sent back. Click here to read more about the way this is done in TDI version of the DA40 and the C172. The wing tanks of the DA42 have a capacity of 26 US Gallons each, for a total usable fuel of 50 US Gallons.

Under normal circumstances, each engine takes fuel from the tank on the side where it is mounted, and sends the unburnt fuel back to the same tank. If something goes wrong (leaking tank, lost engine, …) it is possible to switch each fuel selector to the “CROSSFEED” position. If the left engine is in crossfeed mode, it will pump fuel from the right hand side tank and return fuel to the right hand side tank as well. How to use that smartly ? Here are a few examples.

Example 1: left engine fuel lines get obstructed. By switching the left engine fuel selector to crossfeed, the engine will get and return fuel in the right-hand tank. The fuel in the left-hand tank becomes unusable, reducing the endurance by 50%, but both engines remain operative. With both engines taking fuel from the same tank a level difference develops that can’t be compensated for. The maximal allowed difference mentioned in the AFM is 5 US Gallons. Assuming that tanks were balanced before the obstruction, and that each engine will use 5 USG per hour (thanks to reduced power setting in descent), this limit will be reached within 30 minutes. This means enough time to find an airport and land, but not that much

I’m not sure what would happen when the maximal fuel imbalance limit is exceeded. The fuel has a rather large arm and some aerodynamic forces could be require to compensate the imbalance. And any aerodynamic force generates extra drag and reduces the performance. If one of you (possibly an instructor ?) has more information, a comment would be appreciated.

Example 2: Right engine fails. Because the remaining engine will take its fuel from the left-hand tank only, an imbalance will develop over time. Even with the remaining engine at 100% power, its fuel flow will remain below 8 USG per hour. Flying with an engine inoperative is not exactly a glorious situation (click here to read more about the DA42 single engine performances), but the imbalance is not a problem. By switching the remaining engine to “crossfeed” it will take fuel from the other tank and re-balance the tanks. Flying a DA42 with only one engine is like flying a DA40: the cross-feed pump must be activated from time to time. All fuel on-board remains usable.

The AFM specifies that operating with both fuel selectors in the “CROSSFEED” position is allowed only to test the crossfeed feature (usually while taxiing). I don’t know why for the time being, this is one more question on my list… The fuel selectors have a third position: “OFF”. The “OFF” position is protected by a mechanical safety guard, and I don’t have a lot to say about it…

To be complete, I must add that DA42s can be fitted with optional supplementary fuel tanks. They are mounted in the engine nacelles, each with a capacity of 14 USG. A pair of additional fuel transfer pump is also mounted to tranfer fuel from these supplementary tanks to the main ones in the wings.

As a Piper-pilot I’m already used to monitor fuel and switch tanks periodically. The only C172 I ever flew was TDI equipped and had no “BOTH” position…  Under normal circumstances, the DA42 does not requires any switching and this reduces pilot workload. No pumps to care about, the only possible trick I see is to be sure to switch the correct engine. Left engine on cross-feed means taking fuel from the right-hand tank, not that fuel will be transferred from the right-hand tank to the left-hand one. But after a couple of minutes, there will be more fuel in the left-hand tank. Do you see what I mean ? I promise that I’ll think twice before touching the fuel selectors.