2H4 From United States of America, joined Oct 2004, 8950 posts, RR: 62 Reply 1, posted (8 years 2 months 3 weeks 4 hours ago) and read 4777 times:
AIRLINERS.NET CREW HEAD DATABASE EDITOR
This is from the 172R Information Manual, but it applies to the S model as well:
Operation from either LEFT or RIGHT tank is reserved for cruising flight.
When this fuel selector valve handle is in the BOTH position in cruising flight, unequal fuel flow from each tank may occur if the wings are not maintained exactly level. Resulting wing heaviness can be alleviated gradually by turning the selector valve handle to the tank in the "heavy" wing. It is not practical to measure the time required to consume all of the fuel in one tank, and, after switching to the opposite tank, expect an equal duration from the remaining fuel. The airspace in both fuel tanks is interconnected by a vent line and, therefore, some sloshing of fuel between tanks can be expected when the tanks are nearly full and the wings are not level.
Corey07850 From United States of America, joined Feb 2004, 2519 posts, RR: 5 Reply 3, posted (8 years 2 months 3 weeks 2 hours ago) and read 4747 times:
Quoting SATL382G (reply 2): Isn't it common practice to safety wire this valve to both?
Alias1024 From United States of America, joined Oct 2004, 2644 posts, RR: 2 Reply 4, posted (8 years 2 months 3 weeks 2 hours ago) and read 4742 times:
As said by 2H4, unequal fuel flow is the most common reason.
Another reason would be if you developed some sort of fuel leak out one wing. You wouldn't want the tank to run dry when you are still drawing fuel from it. Air would then be drawn into the fuel lines, and that would ruin you day.
It is a mistake to think you can solve any major problems with just potatoes.
FLY2HMO From , joined Dec 1969, posts, RR: Reply 6, posted (8 years 2 months 2 weeks 6 days 23 hours ago) and read 4715 times:
Embry-Riddle standard operations are to set the fuel selector to either side when parked. This prevents fuel moving from one tank to the other if the ramp is not level, which certainly applies to our ramp
Occasionally the fueling guy won't pay attention to what he is doing and fill the tanks unevenly. This will make the plane bank to the side, which is very annoying. Thus, you put the fuel selector to the heavy tank and burn the extra fuel until the tanks are equal.
777WT From United States of America, joined Jan 2005, 874 posts, RR: 1 Reply 7, posted (8 years 2 months 2 weeks 6 days 21 hours ago) and read 4682 times:
Also if your a newbie and doing alot of traffic pattern and touch n' go's or turns around a point, the higher wing will be used up most of the time.
57AZ From United States of America, joined Nov 2004, 2550 posts, RR: 2 Reply 8, posted (8 years 2 months 2 weeks 4 days 22 hours ago) and read 4585 times:
The requirement for the fuel selector to be set to L or R at shutdown is only for the R and S models I believe. Older 172s can be shut down and secured with the selector set for Both. However, the selector must be set for Both during takeoff and landing operations. Crossfeed valve are more commony used on multi-engine aircraft. We hardly use the crossfeed valve on our 172/R whereas on the 414 it's not uncommon to use it. Reason being that on the 414, the normal procedure is to feed each engine from the tanks on that side. However, slight differences in power setting can create an imbalance. To correct the imbalance the crew can take one of two actions: increase the power setting on the heavier side or crossfeed. On the 400 series twins, there are two individual crossfeed valves. One is the No. 1 engine and the other is No. 2. Crossfeed shutoff (emergency or otherwise) is handled by the Emergency Crossfeed Shutoff valve, located between the No. 1 and No. 2 fuel selectors. Due to the fuel system design on the 400 series, the only way to prevent fuel from moving from the high tank to the low tank on an uneven ramp is to actuate the Emergency Crossfeed Shutoff valve. On our plane, normal practice is to leave this valve in the closed position except during flight operations.
"When a man runs on railroads over half of his lifetime he is fit for nothing else-and at times he don't know that."
Bri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4 Reply 9, posted (8 years 2 months 2 weeks 4 days 16 hours ago) and read 4562 times:
The N and Q models I fly specify to put the fuel selector to one side only during parking, and only when on an uneven surface. It is not prohibited to move it to one side or the other for other flight regimes, but checklists predicate both for engine start, run-up, descent, and landing.
Liamksa From Australia, joined Oct 2001, 308 posts, RR: 0 Reply 10, posted (8 years 2 months 2 weeks 4 days 10 hours ago) and read 4546 times:
We hardly use the crossfeed valve on our 172/R whereas on the 414 it's not uncommon to use it.
Crossfeed on a 172R? You probably don't use it much 'caus it doesn't exist 57AZ . Crossfeed usually refers to the ability to feed the right engine with fuel from the left tank (anb vice versa) for extended flights with 1/INOP.
Trivia: Why to most 172s tend to take more fuel from the left than right tank initially?
N766UA From United States of America, joined Jul 1999, 7991 posts, RR: 27 Reply 11, posted (8 years 2 months 2 weeks 4 days 8 hours ago) and read 4538 times:
It's good practice when you're trying to conserve fuel to switch back and forth regularly over a long flight. Every half hour or so usually does the trick. Also, when fueling the aircraft one should switch to just one tank to prevent any cross feeding to get the maximum amount of fuel possible in the tanks.
727EMflyer From United States of America, joined Mar 2005, 547 posts, RR: 0 Reply 12, posted (8 years 2 months 2 weeks 1 day 19 hours ago) and read 4468 times:
It would be a very bad idea to wire the valve in "both." In an emergency situation you need the ability to select "off" for fire prevention.
2H4 From United States of America, joined Oct 2004, 8950 posts, RR: 62 Reply 13, posted (8 years 2 months 2 weeks 1 day 15 hours ago) and read 4456 times:
AIRLINERS.NET CREW HEAD DATABASE EDITOR
Quoting Liamksa (Reply 10): Trivia: Why to most 172s tend to take more fuel from the left than right tank initially?
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From United States of America, joined Jan 2001, 248 posts, 
From Australia, joined Oct 2001, 308 posts, 
You probably don't use it much 'caus it doesn't exist 57AZ 
. Crossfeed usually refers to the ability to feed the right engine with fuel from the left tank (anb vice versa) for extended flights with 1/INOP.
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