In reality the normally crap fuel gauges do not simply show “zero” and “non zero” but they shake around so much one can’t really read them (IME).
The problem is probably caused by the ball float mechanism which follows the shoshing of the fuel, without adequate damping, Or maybe the damping (prob90 done with capacitors) no longer works because the electrolytic capacitors have dried out over the decades…
Sorry, don’t have time this afternoon to search for it.
Silvaire wrote:
As I mentioned briefly above, certified aircraft are required to have fuel gauges that read empty accurately, and no more.
Do you have a reference for that? I regularly see US sources claim that it is a myth.
I’m not sure what a simple statement like “it is not acceptable” would mean in this context. If the design of a given certified aircraft type does not support the fuel gauges being accurate through their full range, you either accept it or you don’t fly that type of plane – you likely can’t fix it.
My plane has a complex setup of four fuel senders on two tanks, originally certified by German LBA. It works as well as the design allows, but is not very accurate at half fuel. It never will be. Many designs are even worse, as certified, and some are better.
What is “acceptable” in this context is in reality a choice, not a certification requirement nor necessarily indicative of a maintenance issue. If you do choose to fly a type with non-linear fuel gauges and no fuel totalizer it is safe but may be limiting if ‘time on tanks’ is not sufficiently accurate for the planned flight. It seems to me that might be the case if you need to check fuel level accurately half way along a given leg to judge whether to make the only available fuel stop or continue as desired to the destination non-stop.
Just because this is the minimum airworthiness requirement, does not mean it is acceptable for fuel gauges to be in so accurate.
Actually, and I have flown a lot of aircraft, both newer and very old, I have seen a lot of aircraft with quite accurate gauges.
As I mentioned briefly above, certified aircraft are required to have fuel gauges that read empty accurately, and no more. Despite having 1/4, 1/2 and 3/4 marks you cannot expect them to be accurate anywhere else except empty. As a result, ‘time on tanks’ backed up by seeing the gauge moving eventually towards the low end is the only way they can be reliably used to manage fuel in flight and it’s not a problem per se, it is the way they are intended to be used with whatever range limitations that may impose. @Graham’s method is exactly what’s necessary.
A totalizer supplies more reliable fuel level info above empty. Obviously a given plane many have better fuel gauges than type certification required, but you can’t count on it.
Graham wrote:
Ours in the TB10 are Socata original and are appalling – totally useless. They read full when full and empty when empty, but everything in between is a bit random.
My old Cessna 150 was like that. This lead to several precautionary landings with plenty of fuel available.
Some gauges are good, others are appalling.
Ours in the TB10 are Socata original and are appalling – totally useless. They read full when full and empty when empty, but everything in between is a bit random. Every bit has been replaced and it makes no difference. Nothing is faulty, it’s working how it’s meant to work and it’s crap.
It’s no problem for me. My ‘fuel gauge’ consists of knowing how much is in each tank at the start, knowing how fast it’s being burned, and knowing how long I’ve been flying.
I see. Ok, that sounds a bit different.
I have to say after what I am reading here, I am pretty happy with my gauges. 60 years old they may be but they do really fine in comparison.
Mooney_Driver wrote:
Ugh. In which case you’d have to consider that you are flying with inoperative or erratic fuel gauges.
Perhaps I have overstated the issue, because TBH it’s not worse than the factory gauges in this or any other airplane I’ve flown. It’s just that it’s more precise than a simple needle pointing to some relatively ambiguous spot on a scale. It’s the difference between a needle hovering roughly around the 1/4 mark to a precise number of liters ranging from say 22 to 28. In my example, the actual fuel (and that computed by the GI-275 at 60 liters remaining vs the gauges reading a bit over 30 represents less than 10% of the range on each gauge but 50% difference in fuel remaining. On analog gauges one would not even notice this.
So for me the issue isn’t that I have a calibration problem or faulty gauges, it’s that the display shows a level of precision that the simple resistance-based metering mechanism doesn’t support. This extra precision becomes a distraction rather than a benefit, and I would rather just see bar graphs instead of numbers.
