Fuel management on a C172

[S-Works]

Ah DFC making up his own rules again. 3 more posts and it will be in the ANO as law......

My POH FR172K states to fly on both "unless it is desired to completely exhaust a tank in flight , the auxiliary fuel pump will then be required to restart the engine". It also tells me not to run the electric pumps and the engine driven fuel pump at the same time or the engine will stop unless leaned.

So if we add up some off the advise about switching tanks in flight and putting the fuel pump on in my aircraft we are setting ourselves up for a plummet.

So how about this for a thought, read the POH for the aircraft you are flying, there are subtle differences even in the same model across the variants.

[fireflybob]

A quick Google found this:-

RE: C-172 Power Loss at altitude
Airworthiness Directive
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 39
Amendment 39-1415; AD 72-07-02
Airworthiness Directives; CESSNA Models 172, 172A, 172B, 172C, 172D, 172E, 172F, 172G,
172H, 172I, 172K, Airplanes
AGENCY: Federal Aviation Administration, DOT
DATES: Effective March 25, 1972.
72-07-02 CESSNA: Amdt. 39-1415. Applies to the following airplanes:

MODELS SERIAL NUMBERS AFFECTED
172 28000 thru 29999
172 36000 thru 36999
172 46001 thru 46754
172A 46755 thru 47746
172B 17247747 thru 17248734
172C 17248735 thru 17249544
172D 17249545 thru 17250572
172E 17250573 thru 17251822
172F 17251823 thru 17253392
172G 17253393 thru 17254892
172H 17254893 thru 17256512
172I 17256513 thru 17257161
172K 17257162 thru 17258855
Compliance: Required as indicated, unless already accomplished.
To reduce the possibility of engine power interruption at altitudes above 5000 feet caused by vapor formation in the fuel lines, accomplish the following:
(A) Effective now, the airplane must be operated on a single fuel tank immediately upon reaching cruise altitudes above 5000 feet.
(B) On or before April 1, 1972, install at the fuel selector valve applicable Cessna placards P/N's 0509021-1, 0509021-2 or 0509021-3 as provided with Cessna Service Letter SE72-7, dated March 17, 1972, or any FAA-approved equivalent placard which reads as follows: SWITCH TO SINGLE TANK OPERATION IMMEDIATELY UPON REACHING CRUISE ALTITUDES ABOVE 5000 FEET.
(C) Compliance with the provisions of Paragraphs A and B is no longer required when the fuel system has been modified by the installation of applicable Cessna Kit No. SK172-31B or SK172-32 referenced by Cessna Service Letter SE72-7, dated March 17, 1972, or by the accomplishment of any equivalent method approved by the Chief, Engineering and Manufacturing Branch, FAA, Central Region.
This amendment becomes effective March 25, 1972.

Here is the full link:-

C172

[S-Works]

(C) Compliance with the provisions of Paragraphs A and B is no longer required when the fuel system has been modified by the installation of applicable Cessna Kit No. SK172-31B or SK172-32 referenced by Cessna Service Letter SE72-7, dated March 17, 1972, or by the accomplishment of any equivalent method approved by the Chief, Engineering and Manufacturing Branch, FAA, Central Region.
This amendment becomes effective March 25, 1972.

My bold. I am glad mine is french!

[SkyHawk-N]

Those Cessna kits mentioned provide the vent line between the two tanks I mentioned above. As this is all covered by an AD (mandatory) it should no longer be an issue (if ever it was an issue).

[172driver]

The fuel-vent situation brought up by Skyhawk-N is indeed an issue across the Cessna range. I sometimes fly a C182 with rubber tanks and - to the despair of the chief mechanic here - this one simply cannot be brought to draw fuel evenly. Hence this a/c (and, coming to think of it, another 182 I sometimes fly) is flown on the watch hemispheric rule.

[DFC]

It also tells me not to run the electric pumps and the engine driven fuel pump at the same time or the engine will stop unless leaned.

So if we add up some off the advise about switching tanks in flight and putting the fuel pump on in my aircraft we are setting ourselves up for a plummet.

Not if you do what I said in my post.

With so many variations do we teach the common safe operating procedure which will keep you safe in all C172s or do we have the AD status checked on each aircraft hired by a PPL...........Oh and of course watch out for the old C172 that the Placard which requires the selection of L or R immediately in cruise above 5K has faded or has fallen off.

Even you admit that your POH approves the use of Left and Right positions.

Regards,

DFC

[SNS3Guppy]

What's important, here? Arguing over who knows the 172 best? Ensuring that others use your personal technique? Or simply keeping track of your fuel?

There are all kinds of ways you can manage your fuel. Some of those ways are dictated by airworthiness directive, some are by aircraft limitation or manufacturer recommendation, and some are personal preference.

The primary concern is that you keep track of your fuel and keep a way available to make the fuel available. You know, or should know, that your fuel gauges are only required to read 100% accurately when the tanks are empty. With that in mind, it behooves you to watch your fuel useage, time your fuel useage, and time your use of individual tanks. It also behooves you not to push your fuel down to the bare minimums.

A very wise addage is that you shouldn't run out of fuel if you don't burn off the bottom half of the tank. This doesn't mean that you should always have at least a half a tank; it simply means that you should always have a minimum known quantity of fuel aboard which you will not burn. Never violate that minimum number; you decide what's best for you. Don't count on burning one tank dry and then restarting; that may not be an option for you.

Know what you've got on board. You may choose to burn off one tank for an hour and then switch. You may choose to burn off both. You may choose all kinds of methods, but the main thing is that you keep track of your fuel. You should do this in writing; note the times you start, take off, switch, and calculate your fuel separately from simply watching the gauge. Perform reasonablness checks; note that half way through your trip you've burned half of the fuel you intended...if it's going down faster than you think, you have a problem. Same for if it's going down slower...

Know your airplane. Keep track of what you're burning, keep track of your fuel, do it in writing. Other than that, don't worry about what the other guy is doing.

[S-Works]

You are indeed correct my POH does approve the use off left and right positions, but it also makes it clear this is unusual and points out that you may have to use the fuel pumps to restart the engine when it stops due to fuel exhaustion.

I really see no benefit in changing tanks on a fuel system designed to be drawn from both at the same time. Why do you need to know how much fuel is in each tank when you draw from both tanks? It makes no difference that the fuel draws slightly unevenly as Cessna tanks are designed so that in the both position you can empty one tank without problem.

You are as usual coming up with a solution for a problem that does not exist.

We do not expect a PPL hirer to check the AD status, we expect the hirer to ensure that they have access to the POH and that that if they old and faded fuel switch placard is unreadable that it is replaced. We don't expect you to have to make stuff up to fix an imaginary problem.

Edit to Add:

SNS is absolutely right, I said the same thing a few posts ago. Read your POH and operate accordingly. Don't make up stuff ala DFC style.

Personally I use a fuel computer linked to the GNS430 and fly with both tanks on. Over thousands of hours it has proven to be accurate to a fraction of a litre (usually the fractions drained for fuel tests and fillinng my tug!)

[DFC]

I really see no benefit in changing tanks on a fuel system designed to be drawn from both at the same time. Why do you need to know how much fuel is in each tank when you draw from both tanks? It makes no difference that the fuel draws slightly unevenly as Cessna tanks are designed so that in the both position you can empty one tank without problem.

So when that tank empties and you then discover that the reason for it happening was that the other was not providing any fuel your actions are?

Regards,

DFC

[S-Works]

Uh! OK you need to better phrase your question as I don't understand the point of it. I assume you are saying that using your method the engine will stop earlier?

Ok lets have a look at another scenario, changing the fuel selector back and forth every 30 minutes causes it to fail and stick closed between tanks, what do you do? According to my engineer a common failure.

Next?

[stocker]

This may be a daft question but normally only one filler cap is vented to allow air into the tank. What happens when you select the tank that does not have this vent.

[S-Works]

Interesting question! I would assume that it draws air in across the breather pipe that connects the two tanks?

[SkyHawk-N]

I would assume that it draws air in across the breather pipe that connects the two tanks?

Yep, that's what it's purpose is.

Just for interest here is a diagram of the fuel system from the 172 service manual.

[SNS3Guppy]

Why do you need to know how much fuel is in each tank when you draw from both tanks?

What business have you in the cockpit of any airplane if you have to ask such a question? Surely you joke.

[DFC]

Bose,

Uh! OK you need to better phrase your question as I don't understand the point of it. I assume you are saying that using your method the engine will stop earlier?

No.

I asked " What your actions are when you have an engine failure (which unknown to you at the immediate time is due to fuel exhaustion)"

Most people will

a) Fly the aircraft - Best Glide

b) Select a suitable landing area

If there is time available

Select a tank with fuel in it.................

1. Based on the unreliable fuel gauges in the Cessna, please explain how you can tell which of the tanks has the useable fuel in it when both gauges are varying from E to 1/4 in turbulence.

2. How you can determine how much fuel is in that tank

but much more importantly and here is the important point -

In your method when you select the tank with the fuel, the engine will not start because that tank is not providing fuel to the system.

If you reguluarly change tanks then in the scenario I describe the engine will stop the first time you select the blocked tank. Returning the selector to either the Both or the other tank position will make the fuel in the other tank (a known figure) available for diversion.

Ok lets have a look at another scenario, changing the fuel selector back and forth every 30 minutes causes it to fail and stick closed between tanks, what do you do? According to my engineer a common failure.

A situation that has happened on aircraft where it has been operated and left for a long period in the Both position before being moved. A well serviced and reguluarly exercised selector will be reliable.

One has to wonder why Cessna spent all that money on such a feature when in your case they could have used the same design as the C150...........as far less cost and less weight.

You also ignored the following:

You said -

It also tells me not to run the electric pumps and the engine driven fuel pump at the same time or the engine will stop unless leaned.

So if we add up some off the advise about switching tanks in flight and putting the fuel pump on in my aircraft we are setting ourselves up for a plummet.

I say that using my method that will not happen. Do you not agree?

-------

What business have you in the cockpit of any airplane if you have to ask such a question? Surely you joke.

Unfortunately probably not!

One of the most frustrating parts of being a flight instructor must be the idiots who will do one thing while you are teaching them and then totally ignore it when they are let loose on their own - good fuel management and airmanship being one!

Regards,

DFC

[S-Works]

One of the most frustrating parts of being a flight instructor must be the idiots who will do one thing while you are teaching them and then totally ignore it when they are let loose on their own - good fuel management and airmanship being one!

Yes I know. Which is why as an Instructor I teach people to read the POH properly and fly as the manufacturer suggests rather than making stuff up that suits me.

[DFC]

OK then BOSE, as an instructor respond to the points I made in my last post.

Regards,

DFC

[SkyHawk-N]

Most people will

a) Fly the aircraft - Best Glide

b) Select a suitable landing area

If there is time available

Select a tank with fuel in it.................

DFC, for an 'Engine Failure During Flight' my Cessna 172 POH says ...
'Fuel Selctor valve --- Both', not 'Select a tank with fuel in it'.

Based on the unreliable fuel gauges in the Cessna

I've never had a problem with the accuracy of my Cessna fuel gauges.

[DFC]

DFC, for an 'Engine Failure During Flight' my Cessna 172 POH says ...
'Fuel Selctor valve --- Both', not 'Select a tank with fuel in it'.

"Select a tank with fuel in it" is a generic memory item that works for (almost) all aircraft.

In a C172 normally selcting both will select a tank with fuel in it so the above is not different from the POH.

However, the point I am making is that if you cruise on Both all the time and one tank becomes blocked, selecting Both or the tank with fuel in it after the engine stops due to fuel exhaustion will not restore fuel flow.

Regards,

DFC

[FullyFlapped]

However, the point I am making is that if you cruise on Both all the time and one tank becomes blocked, selecting Both or the tank with fuel in it after the engine stops due to fuel exhaustion will not restore fuel flow.

If one tank is blocked, and the other one is empty, then nothing's going to help anyway, is it ? I suppose there's a theoretical advantage in knowing that you've got a blocked tank, but personally, given the nature of the cock, I reckon you're probably at least as likely to bust the thing or have it jam stuck at "Off" as you are to get a tank blockage ...

I used to own a 172N and flew hundreds of hours in it, at all altitudes pretty well up to its service ceiling. Aside from PTO checks, when both tanks would be selected and tested individually, the fuel cock never moved from "Both", as per the POH. No problem. As for accurately knowing how much fuel is on board, I personally never relied on the gauges : I'd know how much was in before take off, and then all the rest is just timing (and always using a max consumption rate whether I was actually consuming at that rate or not).

I now fly a 210, which has no "both" position. I have a bit of a shudder every time I switch tanks, as the selector has to pass through "off" to get to the other tank. Cessna changed this in the next model to include a "both" position ...