Single engine taxi
Join Date: Oct 2003
Location: USA
Age: 53
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Most carriers in the States recommend or require single-engine taxi out to the runway. My carrier had procedures for doing so and we practiced them in the simulator.
It was also very common to shut down the second engine while taxiing out if it becomes apparent that the departure delay will be longer than originally planned. Normally we started the second engine when we were number 3 for takeoff, roughly. But if ATC then slapped a slot time on us we didn't have originally, or the departure was otherwise delayed, we always shut down an engine on the taxiway, and restarted it later. This was quite common and required by the company. You would get poor marks from a check airman if they found that you planned to just sit around waiting for the delay with both engines turning. You were expected to shut one down and restart it later.
There is no requirement at most companies for a mechanic to be anywhere near the airplane, let alone on a headset, during engine start. That may be the rule at some companies, but not most.
A manufacturer may not recommend single-engine taxi, but that is easily overcome by getting the FAA to approve that action. Your company writes a single-engine taxi procedure, the FAA approves it, and presto... you have a legal single-engine taxi procedure. Boeing says it is not recommended for the 757/767, due to the workload. However, single-engine taxi is much easier (lower workload) with the 757/767 than other planes such as the DC-9, which have no company recommendations against it. It seems the lawyers at Boeing just tossed in that prohibition due to pilot workload, even though the workload isn't as high as it is on other airplanes. There is nothing about the airplane's physical abilities which limit single-engine taxi capability. So the FAA doesn't mind signing off on the procedure. Then the airline will usually require that it be done that way, to save fuel.
There is a bit of increased thrust required when taxiing on one engine, but not too much. They key is to use as little thrust as possible, but wait longer for the plane to move. Patience is needed. They plane will take longer to start moving, but it once rolling it will move just fine. If you try to start the plane rolling as rapidly as normal, you will be using too much thrust on the one engine. Also, with the 757, our company recommended taxi on only the left engine instead of the right, in order to power the large engine-drive hydraulic pump for the left system, to improve nosewheel steering. Our company prohibited single engine taxi when the ground was contaminated by ice and snow and so forth. Normally, it worked fine.
Regarding the one engine generator supporting the entire airplane if you shut down one engine after landing while leaving the APU off, it is not a problem. The APU generator normally powers the entire airplane prior to engine start. The APU generator and the engine generators are the same generator on most airplanes. Identical. Therefore, it is not a problem to simply shut down an engine after landing, even with the APU off, then taxi to the gate and switch to external power. Just leave the engine on for the required cool-down time first.
It was also very common to shut down the second engine while taxiing out if it becomes apparent that the departure delay will be longer than originally planned. Normally we started the second engine when we were number 3 for takeoff, roughly. But if ATC then slapped a slot time on us we didn't have originally, or the departure was otherwise delayed, we always shut down an engine on the taxiway, and restarted it later. This was quite common and required by the company. You would get poor marks from a check airman if they found that you planned to just sit around waiting for the delay with both engines turning. You were expected to shut one down and restart it later.
There is no requirement at most companies for a mechanic to be anywhere near the airplane, let alone on a headset, during engine start. That may be the rule at some companies, but not most.
A manufacturer may not recommend single-engine taxi, but that is easily overcome by getting the FAA to approve that action. Your company writes a single-engine taxi procedure, the FAA approves it, and presto... you have a legal single-engine taxi procedure. Boeing says it is not recommended for the 757/767, due to the workload. However, single-engine taxi is much easier (lower workload) with the 757/767 than other planes such as the DC-9, which have no company recommendations against it. It seems the lawyers at Boeing just tossed in that prohibition due to pilot workload, even though the workload isn't as high as it is on other airplanes. There is nothing about the airplane's physical abilities which limit single-engine taxi capability. So the FAA doesn't mind signing off on the procedure. Then the airline will usually require that it be done that way, to save fuel.
There is a bit of increased thrust required when taxiing on one engine, but not too much. They key is to use as little thrust as possible, but wait longer for the plane to move. Patience is needed. They plane will take longer to start moving, but it once rolling it will move just fine. If you try to start the plane rolling as rapidly as normal, you will be using too much thrust on the one engine. Also, with the 757, our company recommended taxi on only the left engine instead of the right, in order to power the large engine-drive hydraulic pump for the left system, to improve nosewheel steering. Our company prohibited single engine taxi when the ground was contaminated by ice and snow and so forth. Normally, it worked fine.
Regarding the one engine generator supporting the entire airplane if you shut down one engine after landing while leaving the APU off, it is not a problem. The APU generator normally powers the entire airplane prior to engine start. The APU generator and the engine generators are the same generator on most airplanes. Identical. Therefore, it is not a problem to simply shut down an engine after landing, even with the APU off, then taxi to the gate and switch to external power. Just leave the engine on for the required cool-down time first.
Last edited by TWApilot; 7th Jul 2008 at 18:55.
Join Date: Oct 2003
Location: USA
Age: 53
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I can offer one suggestion as to why.
In Europe (and most of the world), you must call for a "start" clearance from ATC before starting the engines. They won't give you a start clearance until nearing your slot time. So, once you start up, it is a relatively short taxi time even at big airports.
In the USA, the FAA allows more of a free-for-all, as no start clearance is ever required. You just start engines whenever you like, and call for taxi. Then they put you in line. You might then taxi for 1 or even 2 hours behind an insanely long line of airplanes at a place like JFK or ORD. It is strictly a "first-to-call-for-taxi, first-cleared-for-taxi" situation. You just start up and give them a call to put your name in the hat for the sequence. I can't even count how many times I've taxiied around well in excess of an hour waiting to takeoff, as a result. To do so on two engines would be silly.
I must say Europe has a better system for that sort of thing. You call for engine start, and they just say, "no" if there is a big line ahead of you. We just don't have anything like that in the States. Closest we come is when they give us Slot times, but we don't get those all the time, usually it is the free-for-all program.
In Europe (and most of the world), you must call for a "start" clearance from ATC before starting the engines. They won't give you a start clearance until nearing your slot time. So, once you start up, it is a relatively short taxi time even at big airports.
In the USA, the FAA allows more of a free-for-all, as no start clearance is ever required. You just start engines whenever you like, and call for taxi. Then they put you in line. You might then taxi for 1 or even 2 hours behind an insanely long line of airplanes at a place like JFK or ORD. It is strictly a "first-to-call-for-taxi, first-cleared-for-taxi" situation. You just start up and give them a call to put your name in the hat for the sequence. I can't even count how many times I've taxiied around well in excess of an hour waiting to takeoff, as a result. To do so on two engines would be silly.
I must say Europe has a better system for that sort of thing. You call for engine start, and they just say, "no" if there is a big line ahead of you. We just don't have anything like that in the States. Closest we come is when they give us Slot times, but we don't get those all the time, usually it is the free-for-all program.
Fullwings,
Do you mean you shut down an engine after it had been started when you saw all those aircraft ahead?
Interesting... I had never tought about it... IŽll keep that one to ask the maintenance guys. By the way does your company have anything written about that... another engine cycle Vs fuel saved... I`m just interested to know.
Do you mean you shut down an engine after it had been started when you saw all those aircraft ahead?
Interesting... I had never tought about it... IŽll keep that one to ask the maintenance guys. By the way does your company have anything written about that... another engine cycle Vs fuel saved... I`m just interested to know.
We don't have any formal SOP for s/e taxi yet, so I just shut it down then restart it when required. I like to think I have a fair degree of mechanical sympathy, so that'll be at least a couple of minutes before taking off, to let it stabilise (especially with GE90s). A scan round the cockpit and she's good to go!
I've only gone s/e when confronted with a situation that might use all my taxi and contingency fuel on the ground, thus necessitating a return-to-stand. At the moment I (we) load extra fuel if there are known delays on the taxi out. Engine cycles vs. fuel saved will be a key point if we ever introduce this as an 'economy' SOP, so that'll be up to the bean counters...
