DA-42 NG: Fuel System
by Ghost Aviator
The fuel system on this aeroplane is simple to fly and easy to get wrong on paper, because the crossfeed has firm rules and the examiner tests them. Learn where the fuel lives, how much of it you can actually use, and — most importantly — the do's and don'ts of the selector valves.
3.1 Where the fuel lives
Fuel is carried in an aluminium tank inside each wing, each tank made of three chambers linked by a flexible hose and filled from a single filler on the outboard chamber. Of the fuel in each wing, a small amount is unusable, leaving:
| Tank | Total per side | Usable per side |
|---|---|---|
| Main wing tank (standard) | 26.0 US gal (98.4 l) | 25.0 US gal (94.6 l) |
| Optional auxiliary tank (in each nacelle) | 13.7 US gal (52.0 l) | 13.2 US gal (50.0 l) |
| Total per side with auxiliary fitted | 39.7 US gal (150.4 l) | 38.2 US gal (144.6 l) |
So a standard aeroplane has 50 US gallons usable in total (2 × 25.0), and one with the auxiliary tanks has about 76 US gallons usable (2 × 38.2). The maximum permissible left/right imbalance is 5 US gal (18.9 l).
3.2 How fuel reaches the engine
Normally the left engine draws from the left tank and the right from the right. Each engine has two independent low-pressure electric fuel pumps working in parallel; in normal flight only one runs, and if it loses pressure the ECU automatically switches to the second. From there a high-pressure pump raises the fuel to common-rail pressure and the injectors spray it directly into the cylinders. Fuel that isn't burned is returned to the wing tank.
A FUEL PUMP switch lets you run both pumps together — you do this for take-off and landing, and any time you suspect a fuel-pressure problem.
3.3 The fuel selector valves — read this twice
Each engine has a fuel selector on the centre console with three positions: ON, CROSSFEED and OFF. In normal operation both are ON, each engine feeding from its own side. CROSSFEED lets an engine draw from the opposite tank — you use it to balance fuel during a long single-engine flight. To reach OFF you must twist a safety guard, so you cannot select it by accident.
- Do not take off with a selector in CROSSFEED.
- Do not put both selectors in CROSSFEED at once.
- Do not run the FUEL PUMP together with CROSSFEED — the extra pressure can damage the high-pressure pump and forces special maintenance.
- Do not shut an engine down with the selector — that also risks the high-pressure pump. Shut down with the ENGINE MASTER.
- If an engine is inoperative, its selector goes to OFF.
3.4 The auxiliary tanks (if fitted)
Where auxiliary tanks are installed, an electric AUX PUMP on each side transfers their fuel forward into the related main tank; there are two AUX PUMP switches in the cockpit. Use them together to avoid a lateral imbalance, but switch the two on about 10 seconds apart so the starting current doesn't trip the fuse. Fuel level switches stop the transfer automatically when the auxiliary tank runs empty or the main tank fills. A G1000 advisory tells you a transfer is in progress; a caution warns you when an auxiliary tank is empty and its pump should be turned off.
3.5 Knowing how much you have
Two capacity probes in each main tank feed the fuel quantity to the flight display. For pre-flight you can also confirm the contents mechanically: a communicating-containers measuring device reads the level directly at the tank drain, and a second "fuel indicator II" method uses the aircraft's measured pitch angle together with a lookup table to give an exact figure. Either way, the maximum permitted fuel temperature is 60 °C, and only the approved jet-fuel grades may be used.
Reinforce Chapter 3: DA-42 NG: Fuel System
Test your knowledge and practice actual exam questions for Technical Specific.