LFOB is only good when headwinds don’t change and there’s no return from the fuel pump to the tank (either vapor or fuel line proper). I can see the concept of LFOB useful in airline – jet operations where headwind has a marginal impact on time aloft. But in SEP? A 20 knot change will have a lot of impact on trip time, and unless one is flying over land or familiar (and prepared) with the ETP practice, seeing the LFOB go negative over water won’t help much.
IME, over many flights, I have not yet had a significant change.
Usually the reason is that the winds don’t actually change. But also the maths works in your favour in that if you get a sudden headwind in the last hour of a 5hr flight, its effect on the LFOB is very small – because most of the fuel leading to the LFOB which was computed when the headwind started to be seen had already been burnt.
I have often seen negative LFOB. It’s normal during climb.
I don’t know how you fly, but i make a fuel calculation before I go and then in flight I can compare my LFOB with my calculation and will know if I’m on the safe side or used too much. That’s what the totalizers are great for, for every waypoint you see the LFOB and then you can make your decisions based on that.
for every waypoint you see the LFOB
I suspect you mean “for every waypoint you see the FOB”.
That is how classical fuel planning was done. You had the FOB on the plog, for each WP.
The problem was that without a totaliser you didn’t know how much fuel you actually had 
So the next best thing was having the ETA to each WP and if you were making the planned ETA then you knew the GS was as planned. Still a big assumption but it works if you know your plane.
I don’t do waypoint-based fuel planning. I look at the total distance, disregard tailwinds, take into account headwinds at max, and once in cruise I watch the LFOB.
Peter wrote:
You can, and modern capacitive gauges are accurate to maybe 3% of full scale
Yes capacitive gauges are very accurate… until there’s even a tiny bit of water in the fuel, at which point they will show full even with empty tanks.
That apparently was part of the cause for the police helicopter “engine silence” and subsequent crash at night in Glasgow.
Water has an epsilon_r of around 100.
Peter wrote:
The problem was that without a totaliser you didn’t know how much fuel you actually had
That’s what fuel gauges are for. Totaliser shows you how much fuel flew through the meter. Not how much fuel you actually have. And I think he means that Entegra shows him how much FOB he should have at each waypoint.
Lack of panel space. I’d love one especially since our aux fuel tank doesn’t even have a fuel gauge. But there’s nowhere to put it. Already our radio and transponder is mounted outside of the panel.
I’d also have to imagine with the silly requirement to have two altimeters, there are many planes with bigger panels that don’t have an instrument hole available.
Water has an epsilon_r of around 100.
Is this related?
I cannot believe the capacitive gauges are designed to rely directly on the relative permittivity of the fuel. Well, unless they are truly crap.
They should contain two “capacitor plates” with a fixed dielectric between them, which has an air gap, and the fuel level just gradually fills the air gap. That way the relative permittivity of the fuel is not directly relevant.
Peter wrote:
They should contain two “capacitor plates” with a fixed dielectric between them, which has an air gap, and the fuel level just gradually fills the air gap. That way the relative permittivity of the fuel is not directly relevant.
That’s how you’d do it properly. But hey, this is aviation. The way I read the G-SPAO accident report, the EC135 fuel sensor just consists of two concentric tubes in the fuel tank (with a few slots to weaken them mechanically so they collapse and not pierce the tank in an accident) and they measure the capacitance across those tubes (chapter 1.6.5.1). And AAIB claims this is a typical setup in aviation.
Furthermore, apparently, contaminating water tends to get between the capacitor plates and stick there. According to the AAIB report, about 6% of the EC135 fleet overreads. There’s an alert service bulletin from Airbus Helicopters out about this issue.
As far as I know, even airliners use these probes (concentric tubes). They just have several of them in each fuel tank and their systems should also measure actual relative permittivity of fuel. Even those don’t like water, but I don’t think it’s seen as a weakness, more like a warning that you have water where it’s not supposed to be. Maybe some airliner driver will care to comment.
