Reserve Alarm

[JG3]

Last weekend Dags tested the reserve alarm in his Savannah by shutting off the fuel at the tanks and watched for the alarm to come on. He flew on and on until the engine coughed as the reserve tank ran dry. Of course he was at altitude and the engine recovered as soon as he turned the fuel back on. The alarm had failed to work, so it was lucky that he wasn't really depending upon it.

 

We pulled the float switch out and found it to be totally defective. So maybe a good idea as part of regular preventative maintenance to drain the reserve tank enough to really test the float switch.

 

We all know that the TEST button only tests the lamp, but it still gives a false impression that the system is OK. Better to remove that darn button altogether I reckon...

 

Mind you, the original lamp does need testing, because that tiny bulb has a tendency to work it's way out of the socket. Even when it does come on it doesn't tend to catch the eye in sunlight way over on that side of the panel.

 

So I've replaced the whole lamp assembly with a flashing LED, (Jaycar ZD-1965, $2.45). It's 10mm diameter and really catches the eye when it flashes. It mounts easily in the same hole in the panel a 19mm diameter rubber grommet.

 

I use that alarm frequently on long trips. When using the outer long range tanks I run them until the alarm comes on, then switch to the inner tanks. I also run on time, so have a pretty good idea when those tanks will run dry, and am ready to immediately reach the fuel valve if the engine coughs because the alarm has failed. And of course I only allow that possibility at altitude. If the changeover is due while on approach to a landing, then I switch over early to full inner tanks.

 

A problem that I had for ages was false alarms from that reserve tank. Once an air bubble gets into that tank it can't all escape readily, and a shallow bubble floats around and triggers the float switch. It mostly happened over bad tiger country, ie Blue Mts, Bass St, etc...... Then I removed the 5mm thick spacer between the float switch and the top of the tank. That moved the float switch down by 5mm, so a shallow bubble can now remain without causing false alarms. Works real well now.

 

JG

 

 

[Downunder]

Hello, I'm looking at buying the Savannah kit and those are some really good tips. Can you tell me about the "outer long range tanks"? Any pics?

 

Are they custom or a factory option?

 

Thanks in advance..... Downunder

 

 

[JG3]

Those long range tanks are the standard factory supplied tanks. Many existing Savannahs are so equipped, either at the time of building or added later. They fit easily in the next outer bays of the wing. Some with one extra tank, some with two. With just a pilot and four full tanks still within the weight limit, and an endurance more than 8 hours! That's really handy for inland travel and makes fuel planning easier and more flexible, can stop at a lot of places that don't have fuel. Also makes it much easier and economical to find unleaded instead of Avgas. Some times a dollar a litre difference in price....

 

Savannah definitely the way to go!

 

JG

 

 

[facthunter]

False Indications.

 

A system that is likely to give erroneous indications, is a bad idea. Traditionally fuel qty indicators are notoriously unreliable in aircraft. You need a back-up method of checking your fuel. By law you are not permitted to load fuel on indication alone. ( This may have changed on some modern high tech systems, but there would be some cross checking available there too.

 

You can load from empty a known quantity. You can DIP anytime provided the dip process is proven. You can remove a known quantity from a known FULL tank qty or dipped verified qty. Any thing which follows the rules mentioned would work. You get the idea.

 

If you have left your plane unattended overnight in the open. Some thief may have got some cheap fuel from you. (Caught a few out).

 

You are supposed to use the fuel flow rate from the POH ,but better is the actual fuel flow figure, ( if known for certain). You cannot use the POH figure for planning IF the actual figure is higher.

 

You should use a time/ fuel used log and (unless you are using higher power settings or Carb heat). It should work out pretty right. a proven fuel flow meter is an asset, however it does not cover the situation where you have a fuel leak, obviously.

 

IF you are switching tanks to even up wing quantities and you normally run on two tanks. it is a good idea to wait till you have a fair bit of height or a spot to land, before you select just ONE. (Till you have proven the supply)

 

As a good practice it is good to select BOTH again, well before you enter the circuit. To remind you put a little note on the panel or a rubber band on your finger.

 

Any time you reconfigure the fuel supply , keep your ear tuned for a change in the engine sound. If It appears to be faltering be prepared to switch back to the position that you were previously using, till you sort it out. ( This doesn't apply if you ran the previous tank empty..

 

These are just a few rules I have picked up over the years. Fuel management is a 100% pass subject. nev

 

 

[Kenny]

Reserve light failure

 

Hi Nev

 

are you familiar with the Savannah reserve tank, ie. it is just a 6 litre canister with a float activated dash mounted idiot light that lets you know when all the fuel you have left is what is in the resereve canister. It is nothing to do with monitoring fuel quantities left in the tanks ( Savannah has a simple visual tube for that) Of course one would monitor fuel burn etc as a check to give an indication of when the tank is close to empty , then if it is close to becomming empty and you have sufficent height you would wait for the light to come on before switching back to mains. If close to landing at destination one would switch back to main tanks anyway just in case of a malfunction near the ground and the plane running out of fuel at a critical time,

 

Cheers Dags

 

 

[facthunter]

extra Tank

 

I'm all for a final tank with about 45 mins fuel in it. With a high wing you can generally have gravity feed which is good. You only need a fuel pump in the lower tank to guard against vapour lock, the others are gravity feed and should empty fully in normal circumstances. A 4 tank system is a bit unusual in small aircraft, and when they are well outboard there could exist a fair amount of imbalance. I'm not implying a particular criticism of the Savannah, except for the failure of the reserve indicator and question it's value. My other comments are on fuel system philosophy and handling / management principles in general, and would welcome any other thoughts, comments... Nev

 

 

[eightyknots]

I think I like JG3's solution to the problem of a light not directly in view of the pilot, which is to replace it with a flashing LED.

 

I wonder how long the defective float switch has been in use? I also wonder why ICP chose six litres as a reserve tank (it seems quite small): is there perhaps no more room for something a little larger?

 

Perhaps all older Savannahs should check their float switch for good operation?

 

 

[Guest Walter Buschor]

I run 3 tanks in my Savvy. One tank in one wing and two in the other. Even with a weight difference of 36ltrs plus the weight of a tank it still flys straight as a dye. It seems that this plane can cope well with the extra tank (s) wight and any possible imbalances. IMO if the outer tank (s) are filled up it pays to fly at least for a while on the right inner tank alone to create a little airspace for the fuel return. Otherwise it can lead to fuel leaking from the overflow tube into the right inner tank. Once the inner right tank has empied somewhat I change to the outer until the idiot light flashes at me and then go back to both inners. My plane seems to prefer the lift inner when both inners are turned on.

 

As for the float switch in the holding tank I know of one that leaked . The switch itself sprung a leak . The thread was tight and it leaked through the core and had to be replaced.

 

fly safe

 

Walter

 

 

[ellpar]

I am hoping I have solved the air bubble problem by installing an air bleed line from the top of the 6L tank to the top of the inner right tank. This is also the tank which has the engine return, so this line should eliminate overflow of this tank, as the fuel should feed back down the air line before overflow, regardless of which tank feed line is turned on.

 

Without this any air in the reserve has to bubble back up the feed. I have taps on all 4 tanks.

 

This is untested and only works on paper, has anyone actually tried it?

 

Peter

 

 

[JG3]

The six litre reserve tank is just about right for the Savannah, gives 30 minutes at economy cruise. (This is not to be included in the 45 minute reserve for flight planning.) Sure is a good safety feature. With these large flat bottom tanks it's not possible to know how much flying time left when they are near empty - could be nil or could still be an hour left. Wouldn't want to run to nil and then have to ditch wherever without power. Also would be a pity to do a damaging pre-emptory landing only to find an hour fuel left in the bottom..... The reserve tank allows to continue to the alarm, and maybe get to an airstrip, but if not then still gives 30 minutes to set up for the best outlanding option still with power. This is the first float switch failure I know of, but indicates that it should be part of routine maintenance checks.

 

Gravity flow from the wing tanks in a Savannah is NOT SUFFICIENT for high power flight. I've tested it on the ground, and it's only sufficient for low power settings. Which could be a real trap if the fuel pump is u/s, in that the engine runs fine at taxi/run-up power, but won't be able to hold climb power. There's enough fuel in the carb bowls to start the climb but it will starve suddenly at climb angle while still too low for an easy recovery..... I didn't used to bother with the electric pump on take-off, assuming that gravity would be sufficient, but since I did the tests I always use the electric pump on t/o.

 

JG

 

 

[ellpar]

Many thanks JG, very valuable information. For the aircraft I trained on (not Savannnah) the recomendation was to use the electric pump to start the engine, and then turn it off! This seems crazy to me, I think the engine should be started and run up on the mechanical pump only to check it's pressure, then as you say turn on the electric as a backup for takeoff. At Savannah takeoff angle the wing tanks are not very far above the carburettors, so gravity would not be much help.

 

Peter

 

 

[Guest Walter Buschor]

Hi John,

 

Re: using the electric fuel pup on take off have you bypassed the mechanical pump when doing the tests ? The mechanical pump in mine will deliver a smidge under 5 psi pressure and with the electric pump switched on as well it will go to a full 5 or just a little over.

 

It is my understanding that pressures over 5 psi may overcome the carby floats and if so "drown" the donk with fuel and consequently stop it. ( Bill had that happen to him during a take off run ) .

 

I've always taken the view that if the pressure is good there's juice to keep things moving. If the pressure drops away fix it by switching on the electric pump. That's how I do it in the Sav. I do however always turn on the electric pump before start up to "pressurize" the line. This way I know that it all works.

 

When I used to fly GA we always had to turn on the booster pump on take off and landing no matter what.

 

So back to the electric pump - should we have it on "no matter what" or only if we have low pressure?

 

fly safe

 

Walter

 

 

[JG3]

Yeh, real good point Walter.

 

I didn't the use both pumps in combo for many years because I was afraid of over pressurizing the floats.

 

When my fuel pressure gauge failed I removed it, so now can't tell if the mechanical pump is working, and that's why I've taken to using the electric as well.

 

I guess a better solution is to get a fuel pressure gauge again, and monitor as you do.

 

Point taken.

 

JG

 

 

[ellpar]

From the Rotax installation manual:

 

Fuel Pressure: Max 5.8 psi Min 2.2 Normal 4.4 Delivery pressure of elec pump should not exceed 4.4. Use of electric pump not just as a backup but to prevent vapour formation at high temp or alt. Vapour formation may occur if fuel line temp exceeds 45 deg.

 

 

[facthunter]

Fuel pumps-- Total pressure.

 

A diaphragm type mechanical pump relies on a spring to push the diaphragm up and this spring DETERMINES the pump pressure. IF you add another pump upstream which is higher in pressure, then that pressure will stop the diaphragm coming up, so the mechanical pump is not adding to the fuel pressure at all. Nev