FLIGHT SAFETY article "Twin Trouble"
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Guys,
Thanks a lot for the above discussion - learned more about light twin ops here in half an hour than in a cover to cover read of some textbooks!
Just starting out in light twins after years of being spoilt for performance in fast jets. The concept of not being able to climb away after a single engine failure terrifies me, and all this good guts info is hugely worthwhile.
Thanks again to all for their informed contributions....
SW
Thanks a lot for the above discussion - learned more about light twin ops here in half an hour than in a cover to cover read of some textbooks!
Just starting out in light twins after years of being spoilt for performance in fast jets. The concept of not being able to climb away after a single engine failure terrifies me, and all this good guts info is hugely worthwhile.
Thanks again to all for their informed contributions....
SW
Swingwing
It might help to think of a light twin as a single engine a/c: One where an engine failure has a good chance of 'only' being partial engine failure ie some useable power remaining.
As with any single, partial power doesn't necessarily mean that you will be able to continue flight - but it might.
It's a bit more complicated than a true single due to handling quality deterioration but performance-wise it about covers it...
It might help to think of a light twin as a single engine a/c: One where an engine failure has a good chance of 'only' being partial engine failure ie some useable power remaining.
As with any single, partial power doesn't necessarily mean that you will be able to continue flight - but it might.
It's a bit more complicated than a true single due to handling quality deterioration but performance-wise it about covers it...
Last edited by Tinstaafl; 12th May 2003 at 22:40.
I've always considered an engine failure in a light twin to mean that the height I am at, at the time of the failure, is the best I can hope for.
Take-off, engine fails, look out the window, nup, the tree tops are higher than I am, close both and ditch it straight ahead.
Take-off, engine fails, look out the window, I am 20' above the trees/buildings, OK go for it .. mixture, pitch & power up etc. etc.
Chimbu chuckles, g'day
The five degrees of bank isn't to bring the drag line closer to the longitudinal access. It works like this:
Engine fails, let's say the right one.
Aircraft yaws to the right.
Pilot adds left rudder, and keeps the wings level.
The yawing moment of the engine is now countered by the yawing moment of the rudder, but the side force applied by the rudder isn't countered by anything, so the aircraft accelerates sideways. This acceleration continues until the drag on the body of the aircraft counters the side force provided by the rudder.
Everything stabilises. The wings are level, the balance ball is in the middle, but the aircraft is not flying forward through the air, but forward and to the right (slightly). If you stick a piece of wool on the window of an aircraft you can demonstrate this. i.e. Wool straight up the window on two engines, wool deflected to the left (aircraft slipping to the right) on one engine. Aircraft has a higher drag due to the slip.
By applying 5° (or so, depending on speed etc.) of bank, the side component of the lift now counters the side force of the rudder, and the aircraft flies straight through the air without slip, and with reduced drag. It isn't an old wives' tail!
Take-off, engine fails, look out the window, nup, the tree tops are higher than I am, close both and ditch it straight ahead.
Take-off, engine fails, look out the window, I am 20' above the trees/buildings, OK go for it .. mixture, pitch & power up etc. etc.
Chimbu chuckles, g'day
theory says you then need less rudder because you brought the drag line closer to the longitudinal axis but you have also reduced the verticle component of lift...I tend to think this is one of those things that has passed into folk lore and is perpetuated by the old wives network.
Engine fails, let's say the right one.
Aircraft yaws to the right.
Pilot adds left rudder, and keeps the wings level.
The yawing moment of the engine is now countered by the yawing moment of the rudder, but the side force applied by the rudder isn't countered by anything, so the aircraft accelerates sideways. This acceleration continues until the drag on the body of the aircraft counters the side force provided by the rudder.
Everything stabilises. The wings are level, the balance ball is in the middle, but the aircraft is not flying forward through the air, but forward and to the right (slightly). If you stick a piece of wool on the window of an aircraft you can demonstrate this. i.e. Wool straight up the window on two engines, wool deflected to the left (aircraft slipping to the right) on one engine. Aircraft has a higher drag due to the slip.
By applying 5° (or so, depending on speed etc.) of bank, the side component of the lift now counters the side force of the rudder, and the aircraft flies straight through the air without slip, and with reduced drag. It isn't an old wives' tail!
Bang on Checkboard.
There is a book that has some excellent diagrams of this: 'Multi Engine flight manual for professional pilots' A bit pricey but well worth it.
It explains exactly what was said, but with a few cool piccies of how it happens!
GA Driver
There is a book that has some excellent diagrams of this: 'Multi Engine flight manual for professional pilots' A bit pricey but well worth it.
It explains exactly what was said, but with a few cool piccies of how it happens!
GA Driver
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"Twin trouble"
The practice of keeping any twin on the ground longer than the calculated Vr speed will generate many unknowns, i.e. what will accelerate stop/go be?; where do you calculate the obstacle clearance surface from in the event of the loss of one powerplant? The point is that you JUST DONT KNOW, because you are going outside of the calculatable (Pardon the word) parameters. Flying is not just "seat of the pants". It is also a science with established boundaries for safety. When you venture outside those calculated boundaries you become a test pilot. I would not like to do that unless I knew and understand what I was doing.
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Speaking from experience!Actually the experience came last week I had to shut down a engine after slowly loseing oil pressure.luckily I was at 9500, 50 miles from the closest airport.At 25miles I shut the engine down when oil pressure whent below red line(this was my decision point when i would shut her down)I was empty ecept for 1100 pounds of fuel.On arrival to the circuit I had landing gear probs and had to blow down the gear,which also failed.Did a flybuy of the Tower @1500ft agl,both mains down only R/H with a green light and no nose gear.Quick boot of rudder to the left(dead engside)and bingo 2 mains down and locked.I then decide since I was in the crct that I would restart the L/H eng because I was landing with out my nose gear I wanted one less thing to worry about.Lucky I made this decision as the extra Hyd press from the L/H eng driven pump brought the nose down while turning finals.I have low hours on type but 12 years exp working on 402cs,Soo I now the systems quite well,This all helps when it all turns too pooh!!2 days later a/c repaired(not by me)I got 30 miles out when the L/H whent silent on its own.Feathered and secured flew back to the airport.On a straight in approach aprox 5 mile with heaps of height too burn I extended the gear and as the right hand main locked down I noticed the sky go Red,This was my hydrulic fluid streaming from the resevoir over my windscreenThis time the blow down system worked as advertised.landed the a/c on one engine ,parked it up and whent straight too the nearest pub and got hamered.I think I deserved itThe a/c flew quite well and at no stage did I hammer the live engine,Simply as I was shown ,as soon as I lost the L eng I pushed the R/H throttle back upto climb power and left it too see how the A/C was performing then make adjustments.But I was lucky as I had Height and speed on my side!
gottom 2 dollar
I hope some questions were asked of you maintenance organization by your owner/Chief Pilot or Maintenance Controller. To have that type of S**t happen to you twice in one week in the same aircraft would make me go and buy a lottery ticket. Well done.
I hope some questions were asked of you maintenance organization by your owner/Chief Pilot or Maintenance Controller. To have that type of S**t happen to you twice in one week in the same aircraft would make me go and buy a lottery ticket. Well done.
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404 & Curawong,thanks,the aircraft in question was actually on a delievery flight.It had a 100hrly completed last september by a maintanance company seperate from the the old owners and then sat for 9 months.Im actually a engineer as well and was there when the a/c was checked over after I shut the engine down the first time.No metal in the oil just lots of carbon.the oil system was flushed and engine runs caried out.I think the gear prob was a number of things,tired eng hyd pump on live engine and a/c sitting for to long.The hardest thing was actually closeing the throttle on a engine which was still running except for decreaseing oil press.I should mention It wasnt a indication prob with oil pressure because I was loseing M/P at altitude,caused by lack of oil press to keep the wastegate closed
Props are for boats!
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It also depends on Temp and Density Altitdude and elevation.
I depart max weight at Kingston Sea Level,on a hot day (35c) I use A normal rotation of 95 Knots in the A90. accelerate to blue line 112knots then max rate or angle depending on obsatcles or if I have a SID climb requirement
When I depart at max weight, from Tuxla in Mexico 3741ft elevation in the morning at around 23-26 degrees I roll on thru my 95kts and pull her off at 110kts. You may think Im nuts, but if I dont the stall warning goes off and shes mushy as hell.
Chimbu I tend to agree with on diffferent environs, Stalli I like youre adherence to speeds depanding on terrain clearnce etc.
Practically speaking speed is life and so is height, but density height will allways have the last say on performance especially VYSE.
Keep the blue up chaps
Regards
Sheep
I depart max weight at Kingston Sea Level,on a hot day (35c) I use A normal rotation of 95 Knots in the A90. accelerate to blue line 112knots then max rate or angle depending on obsatcles or if I have a SID climb requirement
When I depart at max weight, from Tuxla in Mexico 3741ft elevation in the morning at around 23-26 degrees I roll on thru my 95kts and pull her off at 110kts. You may think Im nuts, but if I dont the stall warning goes off and shes mushy as hell.
Chimbu I tend to agree with on diffferent environs, Stalli I like youre adherence to speeds depanding on terrain clearnce etc.
Practically speaking speed is life and so is height, but density height will allways have the last say on performance especially VYSE.
Keep the blue up chaps
Regards
Sheep
Moderator
Interesting thoughts in this thread .. my 2c worth .. much of which repeats/summarises bits and pieces from various posts ..
(a) consider going for the appropriate OEI pitch attitude on liftoff for a light twin .. gives a nice AEO acceleration and, if we have an engine failure, control it, do the necessaries and then finesse the resulting airspeed a bit. On higher performance machines, there may be a need to pitch higher to cover the AEO case .. with a need to lower the body attitude during the initial OEI controlling, although this is more relevant to FAR 25 twins.
(b) at low weight/pressure height/OAT, any aircraft should go OK on one. As we approach the other end of the envelope, the available range of speeds to achieve a climb on one reduces markedly. The climb curve is bell-shaped .. if we are above the peak of the bell, then it is downhill all the way until we have a bit of excess grunt left over. For most light twins, at gross weight and a bit above sea level, we need to be somewhere near blue line (as corrected for the conditions of the day) and in the appropriate configuration to get much of the little performance which may be available. Too slow or too fast and we go down rather than up.
First priority for a continued takeoff ought to be stay right way up, then climb or maintain height as possible or appropriate, and then make sure the aircraft misses the rocky bits ... eventually getting back to a suitable runway.
(c) best performance bank OEI is around 2-3 degrees to give zero sideslip .. doesn't matter what multi is being discussed, Concorde included .. wings level is easier to fly for a marginally degraded climb performance .. take your pick. 5 degrees is relevant to Vmca determination and initial OEI control if you are near Vmca .. as the speed picks up, the 5 degree thing is largely irrelevant .. and, if you are near Vmca in a lightie with a failure .. and near to the ground ... then the choice of options is very limited. Climb performance wings level and at 5 degrees generally is similar, so why bother with the additional workload of flying with a bank of 5 degrees for no gain ?
(d) the AFM figures and recommendations are based on the presumption of critical conditions .. ie minimum available runway length and a need to achieve whatever climb performance, if any, that the particular design standard looks for. If you have more benign conditions, then the available range of options increase. Apart from considerations of standardisation (which are very important, especially in a larger operation) ... it is a bit silly, from a risk mitigation viewpoint, to adopt procedures intended for a critical situation as being rigidly appropriate for ALL circumstances where other options may offer a lower risk scenario.
From a simple energy viewpoint, height is more useful than speed .. but energy considerations are only part of the flight management equation...
For instance ... taking off from a 12,000 ft runway with little in the way of terrain to consider ... lifting off at minimum AFM speed, having one quit, experiencing the "oh dear" time delay, slowing down, rolling inverted, experiencing the resulting crash-burn-die scenario is no more/less silly than trying to hold it on until blue line on a short runway with terrain considerations, having one quit just before blue line, unsuccessfully rejecting and falling into the rock-filled gully beyond runway head with a slightly different crash-burn-die scenario ... The best option needs to be considered on a day by day, runway by runway basis.
(e) there are no guarantees at all ... FAR 25 included .. as shown by a succession of crashes over the years ... certification data give us something in the way of reference information to work with .. the outcome on the day depends on a mix of training, knowledge, luck, skill, planning, and keeping cool under pressure.
One thing is certain .. get too slow and over you go .. so the Vmc departure spectre needs to remain very much in focus for a mishandled engine failure and continued takeoff.
The aim is "don't crash".
I haven't flown any light twins for quite some time but, in years gone by, if in a Shrike or Chieftain etc., with a comfortably long (and reasonably smooth) runway, zero flap takeoff with a rotation somewhere approaching blue line seemed a reasonable compromise to me and certainly simplified the continue/reject decision ... none of the old DCA examiners, whom I briefed appropriately as to TOD corrections, conservative assessments of accel-stop distances, etc... ever queried my flight management philosophy on renewals ... (Some aircraft, of course, weren't appropriate for a faster rotation speed.... the dear old Twin Commanche is the best example which springs to mind). Conversely, on a shorter runway, the P-chart Vtoss speed makes much more sense.
My point is that we are faced with death from a number of quarters during a takeoff ... the best solution seeks to stay as far away from all as best as one can orchestrate for that particular takeoff ... generally a one-size-fits-all approach is not the optimum philosophy.
(a) consider going for the appropriate OEI pitch attitude on liftoff for a light twin .. gives a nice AEO acceleration and, if we have an engine failure, control it, do the necessaries and then finesse the resulting airspeed a bit. On higher performance machines, there may be a need to pitch higher to cover the AEO case .. with a need to lower the body attitude during the initial OEI controlling, although this is more relevant to FAR 25 twins.
(b) at low weight/pressure height/OAT, any aircraft should go OK on one. As we approach the other end of the envelope, the available range of speeds to achieve a climb on one reduces markedly. The climb curve is bell-shaped .. if we are above the peak of the bell, then it is downhill all the way until we have a bit of excess grunt left over. For most light twins, at gross weight and a bit above sea level, we need to be somewhere near blue line (as corrected for the conditions of the day) and in the appropriate configuration to get much of the little performance which may be available. Too slow or too fast and we go down rather than up.
First priority for a continued takeoff ought to be stay right way up, then climb or maintain height as possible or appropriate, and then make sure the aircraft misses the rocky bits ... eventually getting back to a suitable runway.
(c) best performance bank OEI is around 2-3 degrees to give zero sideslip .. doesn't matter what multi is being discussed, Concorde included .. wings level is easier to fly for a marginally degraded climb performance .. take your pick. 5 degrees is relevant to Vmca determination and initial OEI control if you are near Vmca .. as the speed picks up, the 5 degree thing is largely irrelevant .. and, if you are near Vmca in a lightie with a failure .. and near to the ground ... then the choice of options is very limited. Climb performance wings level and at 5 degrees generally is similar, so why bother with the additional workload of flying with a bank of 5 degrees for no gain ?
(d) the AFM figures and recommendations are based on the presumption of critical conditions .. ie minimum available runway length and a need to achieve whatever climb performance, if any, that the particular design standard looks for. If you have more benign conditions, then the available range of options increase. Apart from considerations of standardisation (which are very important, especially in a larger operation) ... it is a bit silly, from a risk mitigation viewpoint, to adopt procedures intended for a critical situation as being rigidly appropriate for ALL circumstances where other options may offer a lower risk scenario.
From a simple energy viewpoint, height is more useful than speed .. but energy considerations are only part of the flight management equation...
For instance ... taking off from a 12,000 ft runway with little in the way of terrain to consider ... lifting off at minimum AFM speed, having one quit, experiencing the "oh dear" time delay, slowing down, rolling inverted, experiencing the resulting crash-burn-die scenario is no more/less silly than trying to hold it on until blue line on a short runway with terrain considerations, having one quit just before blue line, unsuccessfully rejecting and falling into the rock-filled gully beyond runway head with a slightly different crash-burn-die scenario ... The best option needs to be considered on a day by day, runway by runway basis.
(e) there are no guarantees at all ... FAR 25 included .. as shown by a succession of crashes over the years ... certification data give us something in the way of reference information to work with .. the outcome on the day depends on a mix of training, knowledge, luck, skill, planning, and keeping cool under pressure.
One thing is certain .. get too slow and over you go .. so the Vmc departure spectre needs to remain very much in focus for a mishandled engine failure and continued takeoff.
The aim is "don't crash".
I haven't flown any light twins for quite some time but, in years gone by, if in a Shrike or Chieftain etc., with a comfortably long (and reasonably smooth) runway, zero flap takeoff with a rotation somewhere approaching blue line seemed a reasonable compromise to me and certainly simplified the continue/reject decision ... none of the old DCA examiners, whom I briefed appropriately as to TOD corrections, conservative assessments of accel-stop distances, etc... ever queried my flight management philosophy on renewals ... (Some aircraft, of course, weren't appropriate for a faster rotation speed.... the dear old Twin Commanche is the best example which springs to mind). Conversely, on a shorter runway, the P-chart Vtoss speed makes much more sense.
My point is that we are faced with death from a number of quarters during a takeoff ... the best solution seeks to stay as far away from all as best as one can orchestrate for that particular takeoff ... generally a one-size-fits-all approach is not the optimum philosophy.
