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The AFM for DA40 states in 4A.3.9 CRUISE checklist:

Electrical fuel pump . . . . . . . . . . . . . . . . . . . ON at high altitudes

CAUTION Operation at high altitudes with the electrical fuel pump OFF may cause vapor bubbles, resulting in intermittent low fuel pressure indications, sometimes followed by high fuel flow indications.

I (sort of) understand the reason for needing to compensate for lower ambient pressure with a mechanical fuel pump, but the problem is I can't find a clear reference to what is considered "high altitudes" for this purpose? It has a demonstrated operating limit of FL164, so we're obviously not talking crazy high.

Is it more of a "if you experience these symptoms, try turning on the fuel pump" type of caution? Or is there some specific altitude above which the pump should always be on?

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  • $\begingroup$ Personal opinion would be that about 10000' ASL would be about "High Altitude" because you definitely need a pressurised cabin above that. $\endgroup$
    – SMS von der Tann
    Commented Apr 27, 2016 at 23:01
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    $\begingroup$ Pressurization is not a requirement for any aircraft, only for certain operations. $\endgroup$
    – rbp
    Commented Apr 27, 2016 at 23:17
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    $\begingroup$ After further analysis, it looks like boiling point (temperature at which the vapor pressure equals the ambient atmospheric pressure) is a key issue related to altitude. Avgas is formulated with a low (6.5 psi) vapor pressure to reduce risk, but above a given altitude vapor bubbles can begin to form. An additional concern is the solubility of gasses in fuel which increases inversely with pressure and can lead to fuel pump cavitation. Question I still haven't been able to answer is - what is the critical altitude, and is it density altitude or pressure altitude to be concerned with? $\endgroup$
    – hemp
    Commented Apr 28, 2016 at 6:01
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    $\begingroup$ @SMS von der Tann: Why do you think you need a pressurized cabin above 10K ft? I routinely cruise at 11K/12K in my Cherokee, since there are mountains hereabouts that tall. (Of course I routinely hike & ski in those mountains, so I'm fairly well adapted to the altitude.) $\endgroup$
    – jamesqf
    Commented Apr 28, 2016 at 6:18
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    $\begingroup$ Pressurization and supplemental oxygen requirements are often confused. Above 10,000 MSL it is required for Part 135 operations to utilize oxygen, whereas for Part 91 the requirements begin at 12,500 MSL. For these, pressurization eliminates the oxygen requirements below certain altitudes. Transport category planes do have their own pressurization requirements, which I think require cabin pressures below 8,000ft at all times. In any case - none of that is particularly relevant to fuel vapor formation and the use of electric fuel pumps. $\endgroup$
    – hemp
    Commented Apr 28, 2016 at 7:14

1 Answer 1

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It seems I've asked a question that has no answer. From the explanation below provided by a Diamond Aircraft representative, it is clarified that the intent of the AFM's caution is not to always run the electric fuel pump at high altitude, but to use it as needed to clear the symptoms - which are more likely to occur as altitude increases.

The IO 360 with the Precision Fuel Injection System does not have a vapor separation and return line to the tank(s). Vapor in the fuel system will travel through the fuel system including flow and pressure transducers, into the injection system. Vapor can form under higher ambient temperatures and low ambient pressures. Fuel gravity fed through the system (electric pump off) from the tanks is “lifted” by the mechanical pump. The suction of the mechanical pump can cause some cavitations (vapor formation), especially as the fuel flows through resistances such as elbow fittings. Vapor bubbles will cause momentary low pressure indications and may also cause momentary erratic or high flow readings as the flow transducer turbine spools up with the loss of fluid resistance. These momentary occurrences can be suppressed with the electric pump. The electric pump activation increases system pressure to compress or return to liquid, the formed fuel vapor bubbles.

[...]

The intention of the steps for electrical pump selection, above each of the Cautions in the AFM is not for continuous operation of the pump, as this could potentially mask a defective mechanical pump and eliminate the redundancy of the fuel delivery system. If the abnormal fuel pressure or flow indications do not stabilize with the electric pump on, emergency procedures and maintenance action may be required.

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  • $\begingroup$ I remain curious about the relationship between Avgas vaporization and altitude. Does bubble formation increase linearly with altitude? Alas I'm not sure I'll ever have an answer to that question, and it does not appear that an answer would change my operating practices regardless. $\endgroup$
    – hemp
    Commented Apr 28, 2016 at 7:05
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    $\begingroup$ Good question and good on you for contacting DA to get an answer! Thanks for coming back to share it with us. $\endgroup$
    – FreeMan
    Commented Apr 28, 2016 at 14:23

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