2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 59
Reply 1, posted (10 years 5 months 21 hours ago) and read 9339 times:
AIRLINERS.NET CREW 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.
Alias1024 From United States of America, joined Oct 2004, 2872 posts, RR: 2
Reply 4, posted (10 years 5 months 19 hours ago) and read 9304 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 (10 years 5 months 16 hours ago) and read 9277 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.
57AZ From United States of America, joined Nov 2004, 2586 posts, RR: 2
Reply 8, posted (10 years 4 months 4 weeks 16 hours ago) and read 9147 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: 3
Reply 9, posted (10 years 4 months 4 weeks 10 hours ago) and read 9124 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 (10 years 4 months 4 weeks 3 hours ago) and read 9108 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, 8593 posts, RR: 23
Reply 11, posted (10 years 4 months 4 weeks 1 hour ago) and read 9100 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.