Questions about 'fuel tank selector' and 'engine shutdown'

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Illi73
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Joined: 04 Mar 2015, 09:12

Questions about 'fuel tank selector' and 'engine shutdown'

Post by Illi73 »

Hi,

I thought this was probably the right place on the forum to ask these questions... I have one question about the fuel tank selector and one about engine shutdown (both related to plane shutdown/shutoff).


Question 1: Fuel selector to LEFT or to OFF when you have parked the plane?

Description: The manual of the Cessna 182 tells to put the fuel selector to "LEFT" when you park the plane and shut it down. Also in videos I see this apparently being 'the way to go'. In the Cessna 172 this is logical, because it does not have an 'OFF' position on the switch, but the Cessna 182 does have an 'OFF' position as well. In comparison to this, the Piper Cherokee has a selector "Left", "Right" and "OFF", where according to the manual, the switch should be placed in the 'OFF' position when the plane is not in use? I understand that these planes have a bit different technical system, and that there can be fuel feedbacks and flows between the wings, etc. but I was wondering, if 'LEFT' is indeed the correct position of the switch when the plane is parked... and why it isn't 'OFF'? Is OFF only used when you really need to cut-off the fuel supply in an emergency or something? The C182 doesn't seem to have a separate fuel cut-off switch like the C172 has...


Question 2: Engine shutdown also appears to be differently described for different planes. While in the Cessna there's instructions in the manual to put the throttle to idle and then pull the mixture out, wait till the blades have stopped and THEN switch the magnetos off... while in the manual of the Mustang (if I recall correctly) there's a specific note that you should switch the engine off by turning the magnetos off and NOT by starving the motor of fuel. So is this again because of a difference in design in the way the mechanical/fuel system of the plane works, or is one way better than the other? Is it, e.g. in the Cessna good to leave some fuel in the motor (like it would be in the Mustang)?

If anyone has any insights into this, it would be welcome. On one way it's really great that these planes are so realistic that they 'work in different ways' and require different steps to handle them, but on the other hand I'm wondering if I'm doing things right because I know too little of the mechanics.

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Great Ozzie
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Re: Questions about 'fuel tank selector' and 'engine shutdow

Post by Great Ozzie »

Hi Illi,

For the C172 / C182, when on the "Securing Airplane" checklist, the manual reads: Fuel Selector Valve - LEFT or RIGHT to prevent cross feeding. The idea being selecting the wing-low side to prevent the fuel from crossfeeding from the high-side tank. The C182's fuel shutoff just a bit different (than the C172) being built into the fuel selector but basically the same idea.

The Cherokee (in practice) I leave on the tank which was last used. No need to move it (not sure where you are seeing move it to off). Normally (for these three airplanes) the OFF position would be used in case of some sort of emergency (inflight fire - off airport landing etc.).

As far as mixture is concerned. No idea about the Mustang's procedure... hazarding a guess would be it's tough enough to start with the lines primed etc. For the GA types like the C172 / C182 / Cherokee, this is certainly not the case. There is a risk of the engine firing (e.g. if the prop were to be moved during preflight) so shutdown involves using the mixture to help ensure that does not happen (by no means a guarantee tho).

-Rob
Rob Osborne
Flight Instructor - CFI, CFII, MEI, MEII
A & P Mechanic


FAASTeam - Safer Skies Through Education
Professionalism in aviation is the pursuit of excellence through discipline, ethical behavior and continuous improvement. NBAA

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AKar
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Re: Questions about 'fuel tank selector' and 'engine shutdow

Post by AKar »

Yes, as Rob describes, the fuel selector position is used to prevent crossfeed from wing to wing. I could think that, per design, selecting an OFF position won't prevent this crossfeed, because the inlet holes in the rotary selector would still be exposed towards the tank lines - just the outlet hole of the valve is blocked by the valve structure. But I don't know, I don't have a good picture at hand, and have never seen the valve taken apart. It depends on how the mixing takes place.

Regarding your second question, I don't know about Mustang, but in small planes you absolutely want to kill your engine by using mixture. Shutting it down by mags could cause some backfiring and even 'dieseling', that is, firing of the engine just due to heated plug head in the chamber. The latter happens obviously uncontrolled in regards to piston and valve position, so it could be more or less unhealthy to the engine. Mustang at least uses much 'colder' plugs than what are used in GA engines, perhaps enough to avoid these issues.

-Esa

Illi73
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Joined: 04 Mar 2015, 09:12

Re: Questions about 'fuel tank selector' and 'engine shutdow

Post by Illi73 »

Okay, that makes sense, thanks guys!

I knew about the 'cross-feeding', but figured that maybe since there was a 'OFF' position, it would also prevent the cross-feeding. Good that you brought up that this may not be the case, and confirming that 'OFF' really indeed is for the emergency situations.

Regarding the Cherokee... I didn't find anything about it in the manual (but it can be that I missed it), its shutdown isn't that well described from what I remember. Also on the 'pilot notes' on the in-sim memopad there's no mention on how to switch the plane off ;) I think that if you press "Dark and Cold" or "Cold start" or something, it might put that switch in the OFF. Also, from videos I found out (for the Cessna 172, which has a separate fuel cutoff) that many airports request or recommend pilots to pull that knob when they exit the plane. Just for safety reasons. So since in the Cessna182 and Cherokee these aren't separate from the fuel tank switch I didn't know what to really do about that. Good to have this cleared up a bit! =)

About the Mixture question: That also makes sense. Thanks!

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