C172N checklist
 

I have just been checked out on a C172N. The Engine Start Checklist provided called for the fuel selector to be on the left tank. The After Start Checklist called for the fuel selector to be switched to right tank. Then the run-up was done on both tanks.

I can't find any information on the internet about it. I've not come across it before on a C172.

Why is this?

Not a type I fly much, and I didn't write that checklist but an inspired guess would be that you've checked both individual feeds then fly the take-off in the setting least prone to any kind of blockage. Seems a fairly reasonable approach for the C172s tank configuration to me.

G

Just looked through my 172 checklists and they all say the same, start on one tank (the lowest), change to the other and then both for power checks and take off. I'm no fuel systems expert but surely it's to check that both tanks are feeding?

The checklist called for starting on the left tank and then switch to right tank after start, in that order. I'm not sure that we would have been on either tank long enough to be certain they were both feeding properly. The take-off was always done on both tanks.

The POH for a K that I fly advises to change to left or right tanks after climb. Our 172 syndicate have the idea that it's the fuel vent on the left wing that acts as an air inlet in the cruise, slightly overpressurising the tank compared with the right and causing it to empty faster. Don't know if this is true, might be urban myth. I fly on alternate tanks anyway, a la PA28, 'coz that's what it says to do.

The POH for the C 172N has the following direction. On the Prestart check list it says to place the fuel selector to "Both". On the Pre takeoff checklist it reminds you to double check the fuel selector is on "Both"

No where does the POH advise the left right both thing on the ground. This is another stupid flight schoolism mindlessly passed down from instructor to instructor.

I have never been able to find a record of an instance where a C 172 spontaneously stopped feeding fuel from one tank. The only instances were flights immediately after the fuel system had been disrupted by major maintenance, usually as a result of a wing removal. In these cases fuel lines blocked offs were not removed when the aircraft was reassembled. Obviously a very careful preflight and extra attention to all aircraft systems are required for the first flight after major aircraft maintenance.

So the result is the check the engine on each fuel tank "check" is designed to identify a problem that never occurs. Unfortunately what it does do is increase the probability that someone will take off after forgetting to reset the selector to both, something I have personally seen on several occasions.

Also if by some one in a million chance, there was a blockage at a tank it could take up to 10 minutes of running at idle power before all the fuel in the lines and carburetor was used up. So if you don't spend that long running on each tank you will have proved nothing.

Bottom line is the check is not required.

I also find it very discouraging that you have an instructor that could not explain why a check was being conducted. This is simply unacceptable. He/She should be able to explain exactly why every check is being preformed and what would be examples of an anomalous outcome.

Thing's 172 syndicate idea
 

You might have just answered an old query of mine. My left hand tank always empties quicker than the right if I leave on "both" in cruise - at any height. 172H. Glad to know that it's not me flying cock-eyed - although that could also be true :p

What Big Pistons said - every word....

Looking up the C172M POH I have to hand (don't have a 172N, but don't imagine there's a signficant difference) I think I've come around to BPF's viewpoint also - although the argument I made originally probably is the reason for the checklist under discussion.

That said, a little story. I was testing an Auster 5, rebuilt after 10 years in a shed, basically the same fuel system as a C172.

I did a 5 minute high power test on L - fine, ditto R, fine. Selected both, ran high power fine. Did taxi tests, launched off to fly a sortie planned to use about 60% the available fuel.

A while later, and about 20 minutes flying time from the airfield, I realised that the valve was drawing from only one side whatever setting I had. A very careful flight back to the field had me on the ground with about 10 minutes of fuel remaining in the tank it was drawing from, and an incipient sense of humour failure.

Which wouldn't have been picked up by this checklist either of course as I'd already exceeded what that included.

G

I've flown the K,M, and N plus a 182P which is basically the same system. I've always noticed that the left hand tank empties faster if you leave it on both. Probably why the POH certainly for the K, can't quite remember for the others. says to switch to one tank or the other for cruise.

The Cessna flight manuals I have reviewed use phraseology like "The use of left or right is reserved for level flight". That is not an instruction to use one tank or the other in level flight, but rather an instruction to not use left or right during maneuvering, takeoff, climb, approach or landing.

If, as the pilot, you feel that you would like to confirm the operation of the fuel system independently, you are free to do so. However, this is not a routine check. To be effective, you're going to have to run the engine for several minutes on each tank before switching, and very certainly more than that minimum number of minutes on "both" before takeoff. It takes a while to run a fuel line dry! If you suspect that the fuel system requires an operational check, than of course you'll also turn the fuel off to assure that the engine stops - right! If you are so uncertain of the proper operation of the fuel system, that you feel you need to check fuel selector operation, and flow from each tank, you probably should refer the aircraft to maintenance staff.

From a safety standpoint, you risk more accidentally taking off on only one tank (or fuel turned off), because you forgot to finish your check, than you're ever likely to turn up doing the check. If Cessna felt this an important check, they would have included it in their checklist, and I have never seen a "change tanks" check in a pilot's preflight checklist. Flying schools who introduce such checks as preflight, in their own home grown checklists are doing a dis-service to pilots by causing them to think these are routinly needed. They are not routinely required in airworthy aircraft (or Cessna would have specified them!).

Received Wisdom and Bullsh*t
 

A very good point. I try to have what I consider a rational explanation for every item on the checks I teach my students. BUT... what if, despite the apparent rationality of what I am teaching, it's WRONG? There's plenty of generic stuff we teach
a) mag check after start to make sure they're both working, mag check on shutdown to make sure they both ground
b) select 1200 rpm when stationary to prevent plug fouling
c) after landing checks - shed electrical load to refresh the battery before shutdown
None of these appear in most POH but have crept into published checklists. Some people have suggested that these 'bloated' checklists full of bullsh*t are there simply to delay departure so schools can make money - also rubbish. They are produced by well-meaning folk who want pilots to operate aircraft safely.

More received wisdom - the left tank vent on Cessna 100 series a/c is 'hidden' behind the strut in relatively stagnant air to reduce the over-pressure in the tank compared with the right.

TOO

I have a vague recollection from many years ago an Engineer telling me the Cessna 172 fuel selector was not a left / right selection but was just a bias towards the selected tank. I.e. If you selected left tank it would draw mainly from the left tank but would still draw a percentage from the right tank and vice versa.

Can anyone shed light on this?

However there are plenty of instances of C172s not feeding from the left tanks as these posts below show in a discussion you took part in!

A 1968 Cessna Skyhawk, Continental O-300-D, in flight, doesn't feed from port wing tank, with fuel selector on Both.

Running on the ground, tank selector to port, no problem. Timed static fuel flow from both tanks is identical.

and these

"A 1968 Cessna Skyhawk, Continental O-300-D, in flight, doesn't feed from port wing tank, with fuel selector on Both. "

How long did you run the test for? My old 172 used to feed from one side for around 45 min then switch itself to the other. I decided never to use "both" so I could keep track of how much was in each tank. Engineering opinion (M3) at the time was that this was normal.

Rod1
flyburg
29th Oct 2014, 10:26
Had the same problem not too long ago, Rented a 1968 C172 in LAX and flew it to SBA. During flight I noticed the left wing tank staying full and the right wing tank decreasing with fuel selector in both. After landing a fuel dip check confirmed that the indicators where accurate. Filled only the right tank and on the way back, same happened. Let the rental place know about it. As most of the flight took place at low altitudes with few places to land I didn't' try and trouble shoot with the fuel selector in different positions!

A first flight of the day check checklist should call for the exercising(not yet mentioned) of the fuel cock and the selection of all positions. Not only does this stop the cock from becoming difficult to use it proves that you can draw fuel from each system and Cessna do provide the left/right cock for fuel balancing in cruise flight. The tank that you are going to use for take off should be the tank that you use for the power check as a restriction in the fuel line will be more pronounced during a power check than a low power taxy.The 172 manual states that only the section of Both should be used tor Take Off

From an instructors teaching point of view, its important that students actually practice operating fuel cocks, especially at schools where the cock is not routinely turned off after flight, otherwise you can be asking a student to turn tbe fuel off in an emegergency situation without ever having shown him how to actually do it and on some a/c turning the fuel cock off can be difficult while flying the aircraft. especially if you have never done it before!

I wouldnt call exercising and/or proving a fuel system stupid or mindless, I would call it good attitude, airmanship and CRM which prevents stupid and mindless mistakes

I very much agree with this sentiment. Students should be taught and practice not so routine pilot actions during their training. I think this extends beyond just fuel cocks to pilot actions like: Shutting down and securing the engine in a glide, feathering and unfeathering a propeller on a twin, gliding a C/S prop aircraft in full coarse pitch, actually using landing gear manual extension controls, running a tank to dry in flight and changing to the remaining fuel, flying with the alternator selected off, effectively discharging a fire extinguisher, properly completing a journey log entry, and perhaps even actually applying something from the first aid kit. None of those should be done by the pilot for their first time, when urgency demands it. But none need be done routinely either! And, in Big Piston's theme, I would hope that the type competent instructor could demonstrate each of these, and explain the factors associated.

Cessnas are known for not feeding fuel evenly - we learn to live with it. If it is frustrating the pilot, they certainly can use the fuel selector to modulate fuel flow as desired. The left wing vent can be out of position, which can affect fuel flow, and flying with a bit of crap can also. This is perhaps more noticeable to Cherokee pilots who are more accustomed to the fuel going exactly the route they select!

I fly a 172P which seems to use fuel from the left faster than the right tank while selected to both.....or at least it did.

I used to have it filled to full in both tanks but now only go to full on the right and 3/4ish full on the left. The reasoning is simple, the vent is on the left tank and when the tank is filled (especially on a sunny day) the fuel pours straight out of the vent as it expands with heat, or as you taxi, hit bumps etc.

I regularly do a three hour round trip and the fuel burn on the outbound leg was higher than the return leg. Since changing my fuelling strategy it has balanced out.

First off we are not talking about "some aircraft" we are talking abut the C 172. The fuel selector is a pretty simple, very accessible handle. If a student needs to continually practice rotating it from the 12 o'lock position to the 6 o'clock position in either direction, than in order to get it right then he/she is not smart neough to be a pilot.

I show the student how the fuel selector works in the cockpit orientation at the beginning of the PPL syllabus. I make them go through the vital actions required for an EFATO before every take off, one of which is point at the fuel selector and say they will tun it to off.

When doing the dual cross country we will select one tank to even the fuel flow because the tanks will never feed evenly and if there is an imbalance the pilot has to continually hold up the heavy wing. This is how Cessna intended the fuel selector to be used. There is IMO, no requirement to add any extra "checks". The ground check is very unlikely to actually identify a fuel feed problem which is in turn a very unlikely event to occur in the first place.

It does however introduce the potential that if the last action of the check is for some reason missed, the aircraft will take off with only one tank selected, an unsafe condition. This is much less likely to occur if the POH procedure is followed

You make my case. In the example quoted doing a ground check of individual
tank feeding would not have identified the problem, which reinforces my point that there is no practical value in the check.

The problem in the airplane above was almost certainly a blocked vent line. Of note Cessna's are supposed to have vented fuel caps on both sides. I occasionally see airplanes with only one vented cap or in one case a plane with solid caps on both sides :uhoh:. This can happen when caps break or wear out and are replaced with a "serviceable" spare :rolleyes:

First off what we are (or should be) be teaching is habit and habit for not just a C172 but for all aircraft that may be flown within the privileges of the rating.

The point that is sometimes missed is that you may want to isolate one of the fuel tanks in the air, if the fuel in that tank was contaminated for instance.

As i know you are smart 'neough' to be a flying instructor you must have misread what I said- I said," a first flight of the day" and I quote:

Pointing out something once at the start of the syllabus is hardly 'best practice' flying instruction is it?


This is true for the C172 but the C152 is only required to have a vent cap on the right side

We should be teaching what is applicable for the aircraft in question. Any differences on other types should be covered in a type check out.

The gravity feed system of C172 is always good for a lengthy tell-tale campfire ...

The C172 has an overhead tube connection between the wing tanks. In the beginning left wing tank had a closed fuel cap and the overspill tube plus open vented fuel cap on right tank. The idea was to vent the left wing tank via overhead join tube and right vented cap. Didn't work out well ;-). They later switched to two vented fuel caps, which did not compensate for unequal tank usage ;-). The overhead join vent tube between the tanks is not exactly on top of the tank, so you always have some partly cross feed (which is the reason why "use only one tank and then you have the same time on the other, is potentially dangerous" = never do that in a C172). On some C172s it can be several gallons cross feed via the overhead join! The changed the tubing later in production and, if memory servers correctly, now now have a curved overhead join tube, which extends above top level of the tanks in the middle, so preventing cross feed.

As there are many different tries of Cessna to work on the tank feed problem, I always advice to use the original check list for that specific built. A C172E is different to a C172N ...