Thrust asymmetry
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Speaking as an ex RAF Flt Eng who flew the Hercules for many years I have never seen, nor heard, of any requirement to use asymmetric thrust for anything other than a 3-engine T/O (ferry flight). The odd exception is 2-engines out or if within a tactical take-off situation. For the purists, there are also some situations whereby the use of asymmetrical thrust might be useful i.e. flying control restrictions. On a day to day basis of normal operations I cannot think of any reason to go "outside of the box" of SOPs.
As previous posters have said, the take-off roll (and subsequent engine failure prior to V1) does not require abnormal NWS inputs. The NWS is obviously utilised during the initial acceleration and prior to rudder effectiveness. If an engine fails prior to V1; immediately close the throttles and commence maximum braking. At Flt Idle, the handling pilot will order "Shut Down No.x Engine". The handling pilot will then select Ground Idle and apply maximum reverse thrust on the remaining symmetrical engines. The handling pilot will then transfer his hand to the NWS tiller whilst the non handling pilot takes the control wheel.
The use of NWS at higher speeds is not recommended.
As previous posters have said, the take-off roll (and subsequent engine failure prior to V1) does not require abnormal NWS inputs. The NWS is obviously utilised during the initial acceleration and prior to rudder effectiveness. If an engine fails prior to V1; immediately close the throttles and commence maximum braking. At Flt Idle, the handling pilot will order "Shut Down No.x Engine". The handling pilot will then select Ground Idle and apply maximum reverse thrust on the remaining symmetrical engines. The handling pilot will then transfer his hand to the NWS tiller whilst the non handling pilot takes the control wheel.
The use of NWS at higher speeds is not recommended.
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Originally Posted by Asquared
Wasn't a bad place really. I was only down here during the winter months, Rest of the time in AK. Nice little bar in the flight school at he end of our Hangar row, and conveniently close to the Iron Horse for the Bike Week craziness.
Last edited by flyboyike; 4th Sep 2013 at 11:26.
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Read about the United 737 runway excursion in KDEN about using rudder inputs on the runway. At high speed bad things can happen. At really low speeds, with an engine failure or one engine not spooling up, it's a quick way to turn approx. 60 degrees to the runway heading.
Hideous to watch, nose of the aircraft bouncing up and down as the tires gain, and then lose, traction with smoke pouring off of the tires.
If you see big skid marks starting at the runway centerline, at the departure end of the runway, and curving towards the side of the runway and then back towards the centerline, you're looking at the evidence of an uneven engine acceleration on takeoff beyond the nose wheels ability to maintain directional control.
Hideous to watch, nose of the aircraft bouncing up and down as the tires gain, and then lose, traction with smoke pouring off of the tires.
If you see big skid marks starting at the runway centerline, at the departure end of the runway, and curving towards the side of the runway and then back towards the centerline, you're looking at the evidence of an uneven engine acceleration on takeoff beyond the nose wheels ability to maintain directional control.
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one way to avoid this problem...make sure your engines are all spooled up evenly before you get moving too fast...and while rolling takeoffs are the norm, on a critical runway (wet or big crosswind) think about holding brakes until engines are spooled up evenly before brake release (not necessarily full thrust, just enough to start you off evenly)
What some people don't realise it that the yawing is made even more so by the fact that the increase in power in a big engine is not linear.
If it was linear, you might only get say continuous 10% difference in thrust. But once the power levers are set to TO thrust, you might get negligible thrust increase for a few seconds then around 80% of the thrust kicks in in only 2 seconds. So if the second engine is 2 seconds behind the first for whatever reason, it's a huge - immediate - difference.
If it was linear, you might only get say continuous 10% difference in thrust. But once the power levers are set to TO thrust, you might get negligible thrust increase for a few seconds then around 80% of the thrust kicks in in only 2 seconds. So if the second engine is 2 seconds behind the first for whatever reason, it's a huge - immediate - difference.
Even though it may not be specific to the EFTO case the OP had in mind, Compressor stall and flarepilot raise a very relevant point, especially in relation to the larger jet engines in pre-FADEC days.
No two engines would accelerate (or decelerate) at exactly the same rate, and IIRC there was (and is?) a certification allowance of about 8 seconds (?) from idle to what is often called TOGA thrust on jet engines. In my limited experience (Conway, Spey, JT3D, CF6), the worse by far was the JT3D as installed on the B707, on which the asymmetry potential is obviously greater than on an a/c with tail-mounted engines.
This differential spool-up time could even catch you out on the B707 if you selected cruise thrust hurriedly after a descent at idle (amusing yaws followed). Most of the delay was in getting the engine to accelerate from idle to the sort of thrust you would use on the approach (about 1.2 EPR). After that, the thrust increase was rapid.
Turning on to the runway centreline for a rolling T/O (standard Boeing technique), the handling pilot "stood-up" the thrust levers for a target of about 1.2 EPR, The F/E monitored the engine acceleration and tweeked one or more levers if necessary. When satisfied that they had all spooled up, he announced "stable", whereupon the pilot advanced them to somewhere just short of where he guessed TOGA would be achieved. Being already spooled up, asymmetry was not normally a problem. (Of course, being an American engine, the thrust-levers could not be pushed fully forward; the F/E had to take over again and modulate the EPRs.)
There was significant potential for a runway excursion if an outboard engine failed during engine acceleration (before the rudder became effective), particularly on a wet runway.
My GUESS (!) is that the Allison turboprop engines on the Herc are already "spooled-up" before the take-off sequence begins, so most of the increase in thrust is achieved by coarsening the airscrew?
As an afterthought to an already long-winded post, I'm not sure how much experience the new generation of jet pilots get in using differential brakes to keep an a/c on the runway on the rare occasions that may be necessary, as vilas has pointed out. On the Dak, it was a vital, everyday tool...
No two engines would accelerate (or decelerate) at exactly the same rate, and IIRC there was (and is?) a certification allowance of about 8 seconds (?) from idle to what is often called TOGA thrust on jet engines. In my limited experience (Conway, Spey, JT3D, CF6), the worse by far was the JT3D as installed on the B707, on which the asymmetry potential is obviously greater than on an a/c with tail-mounted engines.
This differential spool-up time could even catch you out on the B707 if you selected cruise thrust hurriedly after a descent at idle (amusing yaws followed). Most of the delay was in getting the engine to accelerate from idle to the sort of thrust you would use on the approach (about 1.2 EPR). After that, the thrust increase was rapid.
Turning on to the runway centreline for a rolling T/O (standard Boeing technique), the handling pilot "stood-up" the thrust levers for a target of about 1.2 EPR, The F/E monitored the engine acceleration and tweeked one or more levers if necessary. When satisfied that they had all spooled up, he announced "stable", whereupon the pilot advanced them to somewhere just short of where he guessed TOGA would be achieved. Being already spooled up, asymmetry was not normally a problem. (Of course, being an American engine, the thrust-levers could not be pushed fully forward; the F/E had to take over again and modulate the EPRs.)
There was significant potential for a runway excursion if an outboard engine failed during engine acceleration (before the rudder became effective), particularly on a wet runway.
My GUESS (!) is that the Allison turboprop engines on the Herc are already "spooled-up" before the take-off sequence begins, so most of the increase in thrust is achieved by coarsening the airscrew?
As an afterthought to an already long-winded post, I'm not sure how much experience the new generation of jet pilots get in using differential brakes to keep an a/c on the runway on the rare occasions that may be necessary, as vilas has pointed out. On the Dak, it was a vital, everyday tool...
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At a previous employer, we operated 3 times a week into one airport where it was completely common during winter to have a direct crosswind of 30+ knots. This runway was also mostly ice in the wintertime. It wasn't uncommon the gusts to exceed the max demonstrated crosswind component of the airplane I was flying (This was the DC-6, not the L382, but the principle is the same) So, you can either land with full rudder applied, hoping you don't get a last minute gust beyond your ability to control, or you can carry a little more power on your upwind engines into the flare leaving you a little reserve rudder authority. I normally chose the latter. When your runway is slippery ice, your challenges aren't over when you touch down, you still have to keep the airplane pointed down the runway as your rudder authority diminishes with your airspeed, and you have to get stopped, and nosewheel steering and brakes aren't a whole lot of help. So, you work with what you've got. Keeping a little power on the upwind engines helps with the directional control but is counterproductive to getting stopped. On the DC-6, the reverse lockout system required that the throttles were all lined up at the idle stop before they could be pulled into reverse. In the time it takes to bring the throttles to idle, pull the “Martin Bar” which opens the reverse gate (normally done by the engineer) and pull the throttles into the reverse range, 2 things will happen: 1) the airplane weathercocks into the wind and 2) the airplane starts drifting toward the downwind edge of the runway. If at that point, you bring the throttles evenly into reverse, the reverse thrust, in addition to slowing you, will also push you faster toward the downwind edge of the runway. I could draw a little vector diagram of why that happens, or you could take my word, having been there more than a few times that it does happen. The solution is using asymmetric reverse thrust to steer the plane and stop the sideways drift. Now, I don't know if this rises to the level of a “requirement” in your view, but I'm relatively certain that there was more than one occasion where I would have wound up parked off the edge of the runway, had I only used the throttles together as a single unit.
Granted, probably none of this is applicable to jets, but as noted earlier the original question was not specific to jets.
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Probably not, A Squared. it was a reference to the fact that, AFAIK, US engines generally left it up to the operator to avoid "overboosting" an engine on take-off or go-around. That applied, in my experience, from the P&W 1830 radial through to the JT3D turbofan. It may also have applied to the earlier versions of the CF6. I never operated any US turboprops, so cannot comment.
British engines, on the other hand, tended to have a protection against overboost. That certainly applied to the turboprop Dart, as well as the Conway and Spey by-pass jets. You simply firewalled the throttles. I think that may have also applied to large piston engines such as the Merlin and Bristol Hercules, but I stand to be corrected.
British engines, on the other hand, tended to have a protection against overboost. That certainly applied to the turboprop Dart, as well as the Conway and Spey by-pass jets. You simply firewalled the throttles. I think that may have also applied to large piston engines such as the Merlin and Bristol Hercules, but I stand to be corrected.
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That would be more or less consistent with my experience. I've only ever flown US aircraft wih US engines, but with the exception of the normally aspirated ones which can't be over boosted, they all required some retrain. The R-2800 on the DC-6 could easily be overboosted, and the Allison on the L382 is protected from overtemp by the fuel control, but it is certainly possible to over-torque it in cooler temperatures. I wasn't aware that British installations were set up differently
Ahh, the R2800... Never had the pleasure, except as SLF in the late 1950s (DC6B). I think British engineers may have a lower expectation of the skill of operators than their American couinterparts! The R1830 was easily overboosted on a cold day at sea-level.
We certainly were taught to lead slightly with the upwind throttle on a crosswind T/O on the tail-dragging Dak, but I don't remember an equivalent technique during landing.
Heading back towards the original topic, there is one very fundamental difference between an EFTO on a large turbofan and a low by-pass jet engine: the inertia of the big fan (assuming it remains attached) gives you more time to feed in the rudder or differential brake as the thrust decays. Very different also from the situation on pistons and turboprops, which - unless they have an auto-feathering system such as on the Dart - tend to transition from forward thrust to considerable drag in a split second as the prop fines off. (For the uninitiated, the auto-feathering of the propellor/ airscrew would prevent the drag of a windmilling engine. On a jet engine, that is not considered a problem AFAIK.)
We certainly were taught to lead slightly with the upwind throttle on a crosswind T/O on the tail-dragging Dak, but I don't remember an equivalent technique during landing.
Heading back towards the original topic, there is one very fundamental difference between an EFTO on a large turbofan and a low by-pass jet engine: the inertia of the big fan (assuming it remains attached) gives you more time to feed in the rudder or differential brake as the thrust decays. Very different also from the situation on pistons and turboprops, which - unless they have an auto-feathering system such as on the Dart - tend to transition from forward thrust to considerable drag in a split second as the prop fines off. (For the uninitiated, the auto-feathering of the propellor/ airscrew would prevent the drag of a windmilling engine. On a jet engine, that is not considered a problem AFAIK.)
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A squared
You bet none of this applicable to any of the jets I mentioned. While setting power If 747 starts skiddinng all you do is close the thrust and return to dispersal. Also when surface wind is beyond demonstrated and RW ic covered with snow you the only strategy you device is divert. Do I understand correctly that once you taxi out your company expects you to takeoff no matter what?
You bet none of this applicable to any of the jets I mentioned. While setting power If 747 starts skiddinng all you do is close the thrust and return to dispersal. Also when surface wind is beyond demonstrated and RW ic covered with snow you the only strategy you device is divert. Do I understand correctly that once you taxi out your company expects you to takeoff no matter what?
Last edited by vilas; 5th Sep 2013 at 16:08.
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Originally Posted by villas
Do I understand correctly that once you taxi out your company expects you to takeoff no matter what?
The places I have described are villages in the bush, where runway maintenance is spotty at best, and runway friction reports are nonexistent, so you really don't know what you're faced with until your wheels are on it. If you wait until you have a report of mu>40 and light winds, you're going to be waiting a long time. If for no other reason, because the nearest RCR equipment is 300 miles away and there is no road. You'd probably also be horrified to learn that we (both my previous airline and my current one) regularly operate into airstrips on the side of mountains with grades up to 8%, where a go-around is impossible, airstrips for which our stopping distance is 80% percent of available distance instead of the normal 60%, and into frozen lakes and ice runways constructed on the tundra.
All with approval in our Ops Specs. The point being, it's a different operating environment than flying large jets to large urban runways and it's a bit presumptuous of you to attempt to pass judgment. It's a bit like me who has never flown jets, attempting to instruct you on the proper way to fly yours. Know what I mean?
for the latter.
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A Squared
Let me cut the story short. AeroTech asked a question which was not general in nature but specific procedures about Airbus, Boeing etc. I gave him Airbus procedure as prescribed by Airbus. It was not me but you who were presumptuous with no knowledge of jet operations tried to prove me wrong. Except the ice part I am fully aware of mountain flying as I started my flying career in Himalayan mountains in DC3s, where all the hazzards mentioned by you existed, where engine failures were not uncommon. I have also flown turbo props. The procedures you mentioned are not practiced anywhere else. They are of no use to Aero Tech unless he joins your company. It appears more like military flying. Anyway I wish you good luck in what you are doing.
Let me cut the story short. AeroTech asked a question which was not general in nature but specific procedures about Airbus, Boeing etc. I gave him Airbus procedure as prescribed by Airbus. It was not me but you who were presumptuous with no knowledge of jet operations tried to prove me wrong. Except the ice part I am fully aware of mountain flying as I started my flying career in Himalayan mountains in DC3s, where all the hazzards mentioned by you existed, where engine failures were not uncommon. I have also flown turbo props. The procedures you mentioned are not practiced anywhere else. They are of no use to Aero Tech unless he joins your company. It appears more like military flying. Anyway I wish you good luck in what you are doing.
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Odd. Here's a video of pilots doing exactly that. In a 4 engine jet, no less.
I get the impression he is a new pilot. If he were to wind up flying a Navajo (just for example) in the North, He'll find some of the techniques useful. If you re-read the tread, I'm not the only one here who has made use of asymmetrical thrust on occasion, nor am I the only one who read the OP as general question vs specifically jets. And I'm not the only thread participant who has operated on ice. There's at least one other, and I believe I see aircraft he flew in the video I posted.
The bottom line is that asymmetric thrust can be a very useful tool, and to make blanket statement that it's just not done is inaccurate.
Last edited by A Squared; 6th Sep 2013 at 03:14.
