Blue line - not best airspeed on 1 engine?

[aircraft]

Hi all,

Why is the single engine best rate of climb airspeed (vyse) shown on the ASI with a blue line? Why is not the vxse marked?

Theory says that, if you lose an engine on takeoff and have obstacles to clear, you are better off at the single engine best ANGLE of climb speed (vxse).

I can't think of why RATE of climb would be important in an engine out case, but can easily think of why ANGLE of climb would be important.

What is the answer? To help in your deliberations, bear in mind that vyse is the speed at which you have the most excess POWER; vxse is the speed at which you have the most excess THRUST.

[Chimbu chuckles]

Well how many times have you seen TORA=TODA?

Me I'll carry the extra speed allowing less rudder and giving me the maximum control and best chance of surviving the experience.

[aircraft]

You say flying at blue line gives you the "best chance of surviving the experience" - that may not be true if there are obstacles ahead.

In fact, a perception that you are not going to clear the obstacle may cause you to start making turns. What does turning do for your (marginal) performance?

[maxgrad]

Pre Take Off Safety Brief
Done prior to every takeoff, for that takeoff. Taking into consideration, a/c perf, Wx, pilot skill, time of day, LOCATION.

We all do this or should anyway.
Each brief will have differences depending on variables. Prior to takeoff I will look at the terrain with outside reference and on my charts. If high ground is an issue I will take it into account for that take off. If I can use a different option,(say a t/o in the opposite direction, wind and other factors permitting), I will take it and accept the higher VYSE.

edit
T/O out of YTNK in a busted @rse PA31. engine fail after takeoff RWY07. fase ones ane slight right hand turn to miss hills and crash land on flat open area. Yes turns stuff up the performance but sometimes you have to turn. Wouldn't it be better to make a turn with more performance on your side?

[john_tullamarine]

Points to ponder ..

(a) for lighties at reasonable weight, performance is marginal regardless.

(b) blue line is a useful target at gross weight. At lower weights, the speed range for which climb may be possible expands a little

(c) if you are worried about clearing an obstacle after takeoff OEI .. then possibly/probably you are better off putting it back on the runway and, at least, having an opportunity of walking away from the wreck.

(d) if you are operating off a short runway with tiger country around, then you MUST accept a higher risk for light multi operation than if you are operating from a major airport with no obstacles to speak of ..

(e) eyeballing obstacles pre-takeoff is an exercise in wishful thinking and has little basis in reality

(f) shallow turn (say, 10-15 degree bank) is worth around 0.5 - 0.7 percent climb gradient loss depending on how you do the sums.

(g) better energy management to maximise height ... hence a better technique to climb out AEO at best ROC to maximise height before the OEI bit starts ... should that be your lot on the particular occasion.

[RENURPP]

If the question is why is it on the airspeed indicator, I can only guess and here it is.

Unless the engine fails at lift off, no runway remaining and your decision is to continue and possibly that would be best done at angle of climb. This is the only instance that I can think of that I would climb at this speed.

At any other time Vyse would be the most aprropriate speed. Even if it is in the cruise and you want minimum descent rate. So as the most appropriate speed in , most cases it would make sense to placard it. Having said that Vyse changes with weight so the placarded speed is not always correct either.

My logic anyway

[aircraft]

RENURRP,

Thanks for the reply, and thanks for trying to actually answer the question.

You have got me thinking that the reason it is marked, is to give you a speed to fly should you lose one in cruise. If you are going down, then this speed will give you the most amount of time remaining in the air.

If you are going down into the sea or jungle then that extra time could be very important.

To really find the answer to this question, you would need to delve into the history of multi engine flight/certification. Perhaps it was during the early days of big radial engines, when engine failures were much more common, that someone had the bright idea of trying to maximise the remaining air time.

Maybe it has some origin during wartime, when the extra time aloft could make the difference between whether the occupants all manage to bail out or not.

I am wondering this about blue line because I suspect that modern twin training is giving this speed undeserved emphasis - to the point where pilots single mindedly plan this speed without any thought to vxse (I know a couple of pilots that believe that, if you are going to climb, it will be at blue line but not a knot below).

I will try to research further and post my findings back to this forum.

[Bombay]

Couldn't agree more. Vxse would be far more useful in most circumstances.

As for the arguement that Vyse is there for driftdown reasons: That may indeed be true but why bother mark it on the ASI?

We don't mark Best Glide Speed on the ASI in singles, yet we are expected to stick to it doggedly in the event of an engine failure.

Furthermore, Best Glide Speed varies, as does Vyse. So why do we bother marking Vyse on the ASI?

Bombay

[Woomera]

john_tullamarine eeerm you and me both me old!

The Radio mast off the end of EN 26 always got my undivided attention with a predetermined decision as the most likely side to pass it on, provided that I was still proceeding at the time.:D

[john_tullamarine]

Some more points to ponder ...

(a) requirements ? FAR 23.1545 (b) (5) (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=0f556e77ab4dd533e5fa049b1c76ed06&rgn=div8&view=text&node=14:1.0.1.3.9.7.105.16&idno=14), read in conjunction with FAR 23.69 (b) (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=0f556e77ab4dd533e5fa049b1c76ed06&rgn=div8&view=text&node=14:1.0.1.3.9.2.60.19&idno=14) might be interesting.

(b) an interesting .ppt slide show (www.faa.gov/fsdo/orl/files/pp/multiengine.ppt) from the FAA website. This is not endorsed either by PPRuNe or the moderators but is noted for your interest and convenience.

(c) generally, those who worry about best climb and best angle OEI in lightie twins, as contrasted with just best climb rate for the circumstances ... often don't have much/any time in such beasts ?

Most of us are ecstatic if the beast climbs at all on one ...

There is a major training expectation problem .. training usually is done at light weight and, of course, the bird will climb. In the real, dollar-driven dirty, nasty world of work, however, one carries payload ... and the bird is far less likely to leap to the heavens OEI ... If one believes the tales of some, sometimes some carry more payload than one might consider to be appropriate ...


Woomera, good sir ... second that comment.

.. reminds me of another exercise as well .. years ago (once upon a time, as they say), I elected to execute an AEO miss from the flare in an empty (just me and not much gas on board) Shrike from EN 26 using a derated takeoff power setting of my choosing (I don't recall that there was any good reason for it .. just seemed like a good idea at the time).

Convinced me of the value of using all the available ergs from that time hence... did get a good look at the far fence, though .. point of the tale is that light twins need all the help one can give them to claw their way to the heavens ...

[aircraft]

john_tullamarine,

Thankyou for posting the references to the FARs. I was unable to access the ppt file though, so any chance of a quick run down on it?

What caused me to make the original posting was the idea that:

1. Blue line is not the best speed to climb at if you lose one on takeoff, and
2. Modern twin training is emphasising blue line to such an extent that pilots, after a while, forget about VXSE and why it exists.

So, coming back to the original question - why is blue line marked on the ASI? Or, rephrased, why would anybody want to use it?

For OEI climb after takeoff, it is not the best speed from the point of view of survivability so I doubt that it is there for the OEI takeoff case - more on this below.

The only truly beneficial thing this speed brings is a maximum time aloft (if going down) having lost an engine in cruise (and interestingly, the FAR 23.69(b) you quoted refers to enroute climb/descent).

If you use this speed in the OEI takeoff case, then (theoretically)you have the best climb with respect to time.

But, climb with respect to time is not critical and cannot kill you, whereas climb with respect to obstacles is and can. You don't need to be a pilot, or have any multi engined experience at all to work this out - an Accountant could tell you this.

So is it possible that the blue line is there to guide you in the OEI takeoff case? I don't think so.

If it is there for this situation then they could have chosen a better speed. VXSE is a better speed, but if concerned that this speed is getting too close to VMCA, or is too far back on the power curve, then how about a speed that is a small margin above VXSE? (VYSE is way above VXSE in most lighties).

Furthermore, given that the optimum speed varies with weight, they could have marked the range of speeds with a small arc.

In my opinion, the circumstantial evidence supports the case that the blue line is NOT there to help you in the OEI takeoff.

Although off the point, my thoughts on the general subject of how to conduct the OEI takeoff:

Given the desperately marginal performance of most lighties in the case of the OEI takeoff, I can't help thinking that every little thing you can do to improve your situation will add up to make the difference in whether you clear the obstacle or not.

One of those things you can do is to NOT make any turns - which immediately means that obstacle clearance can only be achieved by climbing. Don't assume that "a few little heading changes" here and there will always be an available option.

Climbing at VXSE instead of VYSE could make the difference between whether you clear the obstacle or not - and that difference could be only 5 feet.

[john_tullamarine]

aircraft ...

(a) ppt file - the link should give you the option of downloading and saving the file. Then run it in Powerpoint. If that doesn't work, let me know and I will email it to you ... on a different computer at present so not too sure what the file size was without downloading it again. The file gives a bit of an overview. Only had the one look at it earlier but, on face value, it seems to have a sensible message.

(b) "Blue line is not the best speed to climb at ... " Consider that a typical light twin, especially under 6000 lb certification, is more likely not to climb at all with any decent sort of gross weight. The speed range for any climb may be very narrow if, indeed, it exists. At higher weights, and especially at higher density heights, it may well be the case that we are looking at minimising descent rates .. speed variation from optimum just increases the descent rate.

You are quite correct in suggesting that blue line doesn't have a great deal to do with takeoff failures... takeoff failures aren't a serious consideration in this class of aircraft. The design standards are more concerned with takeoff failure controllability and, by inference, maximising the potential for a semi-successful crash landing ... The old adage that one ought to view a light twin as a generally more useful single than a real single has a lot to recommend its philosophical adoption for most twins.

The higher performance twins are in a different situation but, for these, the commercial marketing compromise leans toward cruise speed marketing and the higher power/weight figures help in the takeoff failure case ... provided one's currency and basic skills avoid the Vmc departure excursion ...

(c) "Modern twin training is emphasising blue line to such an extent ...". Climb angle speed is not much use if there is no climb or little climb capability. Keep in mind that blue line is a specific case optimum and needs to be varied a tad to account for weight and height variation.

There is the specific case where this argument might be varied and that is where there is a small obstacle in the immediate after takeoff area .. in this case, the better case might be to drag the aircraft over the obstacle and then go for the acceleration after clearing the obstacle .. an airfield boundary fence might be a good example. Still very tenuous, though.

A similar argument holds for high energy rotor helicopters where a tactical technique is to overpitch the rotor to clear a close-in critical obstacle adjacent to the LZ ... if the area immediately beyond is not clear, then it's into the trees you go. First time I saw this technique as a youngster in an Iroquois opened my eyes a tad ... Apparently a not uncommon technique employed in Vietnam to get out of hot LZ situations ..

It is important to emphasise the philosophical point that, in the event of a failure during the takeoff and prior to achieving cruise configuration and a sensible speed .. the pilot is on his/her own .. successful resolution is as much a case of good luck as good management for realistic payload cases.

(d) " .. why would anybody want to use it? " .. optimum climb rate speed gives you the best chance of getting away from the ground .. once you are configured and at the speed. How you get to that circumstance is another consideration altogether.

It is probably best not to dwell too much on the takeoff case, as continued takeoff is not a required certification case for the standard FAR23 bird. The consideration is more geared to the successful recovery from a cruise failure. The flight time between a minimum speed schedule takeoff failure and achieving cruise climb OEI conditions is a real grey area and you are on your own to a large extent.

(e) "For OEI climb after takeoff, it is not the best speed from the point of view of survivability so I doubt that it is there for the OEI takeoff case ..". Blue line is most certainly not predicated on a takeoff failure consideration and is predicated on a cruise configuration.

For an energy and crash survivability consideration, other things being equal, one is better off putting the aircraft back on the ground at a lower speed and under control .. this may even entail closing the operating engine throttle and treating the exercise as one would with a single engine aircraft failure in similar circumstances.

(f) "The only truly beneficial thing this speed brings is a maximum time aloft .. ". No argument. However, for the takeoff failure case, for many aircraft in most cases, a speed other than at or near optimum climb rate will only serve to increase the chances of a return to Mother Earth. If you are necessarily going to crash, then one only wants to remain under control and keep speed as low as is sensibly practicable. This means, naturally, avoiding an asymmetric high thrust, low speed, Vmc approach case ...

(g) " .. climb with respect to time is not critical and cannot kill you.." .. but no climb may do just this just as effectively. Unless the circumstances on the day are very favourable, continued takeoff following a failure generally is not the best option. Add to this the likelihood of a low currency pilot ending wrong side up on the way to the ground in a Vmc departure upset.

(h) ".. then how about a speed that is a small margin above VXSE? ". Fine, but not declared in general. No reason why one cannot run a controlled test card series to arrive at a more useful speed, if such a speed (range) might exist under nominated conditions ...

(i) " .. every little thing you can do to improve your situation will add up to make the difference in whether you clear the obstacle or not". If there is an obstacle and you are off a short strip, then the better strategy in most, if not all cases, is to put it back on the ground and go for the lower energy crash scenario .. from which one might be lucky enough to walk ..

(j) " one of those things you can do is to NOT make any turns .. ". Again, if you have anything other than a benign obstacle situation, climbing (turning or not) is tenuous. If the decision to continue is better than putting it back on the ground, then a gradient loss due to a climbing (descending ?) turn may or may not be preferable to a straight flight path depending on the sums in the circumstances. No different to a 20.7.1b net flight path planning exercise consideration.

(k) " climbing at VXSE instead of VYSE could make the difference between whether you clear the obstacle or not .." or hitting the ground a lot sooner on the way to the obstacle .. as the case may be.


If should be obvious that the takeoff failure considerations are not amenable to strict protocol other than for the matter of standardisation which has its own benefits in a high stress situation.

The basic error lies in presuming that this class of aircraft necessarily will have a climb capability. Certification reality and hard-won knowledge in the field is that this certainly is NOT the case ...

I always err toward the "put it back on the ground" strategy unless I am in the fortunate situation of having a very benign obstacle, weight and elevation scenario.

Indeed, if I am in a low weight, longer runway situation, I will do the basic sums and choose to stay on the ground until I get to the blue line equivalent for the circumstances. Makes the failure decision process much easier ...

Interestingly, never had an IRE argue with my strategy .. especially when my brief includes something along the lines of "any simulated failure below xx ft will be treated as real and I WILL put the aircraft back on the ground".

That's not to say that other pilots can't have a different strategy in this class of aircraft and, to be honest, I have never been put to the test in this class.

Above all, one MUST NOT confuse FAR23 (lack of) capability with the very much higher redundancy capability of FAR25 designs. The two are chalk and cheese .. and even FAR25 birds don't come with a guarantee ... just a much higher probability of a given outcome ...

If your FAR23 machine is operating at low weight, low altitude, and low OAT .. and you have sensible data, of course you can go for the Vx scenario ... this might be useful for the owner-pilot in a higher performance twin but is not a great deal of value for the normal commercial light twin operation.

Consider, also, that many such owner-pilots might just fall into the lower currency group and end up in the twinkle-roll-into-the-ground trick. Just in Oz, alone, we have seen this happen in several accidents involving higher performance light twins.

Important to keep in mind that the real world is full of surprises. If the pilot is in very good currency, with plenty of OEI practice exposure (easy enough for the FAR25 pilot with simulator training access but not so for the typical FAR23 case) he/she has a considerable advantage.

The best strategy is to load the dice in favour of a successful high probability outcome .. and this might well vary according to circumstances.

I guess we shall have to continue to agree to disagree ....

[aircraft]

john_tullamarine,

Thankyou for your post.

You signed off with the comment "I guess we shall have to continue to agree to disagree ...." but I feel that we are actually substantially in agreement - the only possible disagreement was in the nature of "the obstacle", but this is understandable, given that there has been no detailed discussion on this.

Thankyou, once again, for taking the time to enlighten us with your thoughts.

Thankyou to all the other posters.

Best regards

[Tinstaafl]

Aircraft, it seems to me you're presuming the a/c will have some performance in the after take-off OEI case. That's not really the case. Also, any performance specifed is 'guaranteed' (to the extent one could consider it a guarantee...) onlyafter the a/c has been configured ie gear & flap up, feathered, cowl flaps as specified, max power on the good engine etc etc. This is very unlikely to be the case in an obstacle limited take-off.

FAR23 specifies two classes, below & above 6000lb MTOW.

<6000lb & the only performance the aircraft needs to achieve - other than controllability issues - is for its climb performance 'be established'. That doesn't imply the performance must be a positive number!

>6000lb & there's a very small climb rate required proportional to stall speed.

Obstacle or not, shortly after take-off the best option may well be a controlled landing/crash - just like in a single - and not an attempt to continue flight. Vyse is the most efficient use of the available energy which may help you reach a more suitable forced landing site.

[aircraft]

Tinstaafl,

Thanks for the reply.

You say "Vyse is the most efficient use of the available energy which may help you reach a more suitable forced landing site."

I don't agree that VYSE is the speed most useful for covering the greatest distance on descent.

If we are talking about our climb (or descent) with respect to the ground, then we are talking about an ANGLE.

If we are talking about our climb (or descent) with respect to time, then we are talking about a RATE.

When you refer to reaching a suitable landing site then clearly you are referring to the ANGLE of most shallow descent.

Flying at VYSE in the case of a descent towards the ground simply maximises the remaining TIME aloft - which, as has been established in earlier posts on this topic, could well be your goal.

Best regards

[Blue Sky Baron]

Although its benn 3 years now since I've had the pleasure of p[lacing my sizeable rump in my beloved Baron, if my memory serves me correctly I was always taught that it was the ATTITUDE that was most important on a OEI climb. "Keep the nose attitude at 10 degrees and you will obtain the best climb" is what I recall, howver I will have to look it up again to be sure.

BSB

[Captain Sand Dune]

Don't have a lot of time on light twins. Have always considered their single engine performance suspect as best.
Therefore I generally treat it as a single engine take-off ie if there's runway remaining when an engine goes, I'm putting it back on.
If no runway left, I'll go for Vysein order to keep a better margin above Vmca.
If obstacles then become a problem, I'll bring it back to Vxse.
If all else fails, it's back to the single engine technique of finding something soft to crash in!

[aircraft]

Frickman,

My God. I hope you are not a practising pilot because if you are, you are a walking case of negligence.

Blue line speed is not where you have the best lift/drag ratio but would be near there (also the case for VXSE).

Blue line is where the excess of thrust power available over thrust power required is greatest.

VXSE is the speed where the excess of thrust available over thrust required is greatest.

So not much about lift but plenty about thrust, power and drag.

As for your statement:

"So to fly any faster or slower, as in the case of a fictitious VXSE all you'd do is increase the drag = decreased performance = Crash kill burn."

I suggest you urgently advise the airworthiness authorities in Australia, the USA, and everywhere else in the world that they are trying to kill people (poor misguided fools).

Also, advise Cessna, Beechcraft, etc that there is no such thing as VXSE so they should stop instructing pilots to fly it when there are obstacles to clear.

[john_tullamarine]

aircraft,

I suspect that we are worlds apart philosophically ...

Looking at your profile, and to put things into perspective, are your thoughts and agenda based on Duke experience (nice toy - half your luck to have one to play with) or the other end of the light twin marketplace (good workhorses but not much grunt to spare) ?

Higher performance aircraft, such as the Duke, offer far more options to the pilot than does, for example, the venerable workhorse Aztec ....

Those who fly the majority end of the market really don't have much option in the nasty event of a failure during the grey area period prior to the aircraft's being established in a cruise climb configuration ..... concern over best angle and best rate speeds becomes illusory at best ...

The OEM agenda is based substantially on minimisation of legal risk ... your immediately previous comments need to be read in such light ..

[aircraft]

j_t,

I agree that minimisation of legal risk is a large part of the OEM agenda these days, but I don't believe it would have been in the days when most of the light twins were built.

My view that VXSE may often be a more appropriate speed than VYSE stems from the idea that, given how marginal the performance is, and how desperate the situation when you lose one just after takeoff, you need to do *everything* possible to maximise your chances of surviving.

Making it easy for you to know what things you need to do is a page in the Pilot's Operating Handbook (POH).

That page in the POH tells you to do things like:
- Fly at VXSE until obstacles are cleared;
- Adopt a bank angle of 5 degrees towards the live engine;
- Trim for balance ball half a width towards the live engine;

This is just a few things but you get the idea.

So, lets say you lose one just after takeoff. What do you do? Follow the manufacturers instructions or follow your own "DIY" set of instructions?

If the former, then you follow them *exactly*, otherwise you are more in the latter category.

Following the manufacturers instructions precisely may not be as easy as it sounds - look again at the list above, which was taken from the POH for a twin currently flying in Australia.

Notice that it says to *trim* for balance ball half a width towards the live engine - it does not just say "position the balance ball..." or "apply rudder sufficient to...". It says to TRIM, so that is what you must do - but how many of us would instead think that it was sufficient (initially) just to hold the rudder pressure with the foot?

There is a very good reason why the manufacturer is saying to TRIM, but if you ignore this (even unintentionally), what other things are you also ignoring?

Now, what about that obstacle that you should studiously be flying at VXSE to avoid?

When you mention the word "obstacle", most people conjure up an image of a dirty great tower (or tree) off the end of the runway.

Well, the kind of obstacle that can bring you undone may be a lot more subtle than that - you may not even be able to see it whilst you are sitting on the runway - a line of trees a few hundred feet AGL and several kilometres away, for example.

You might think you can turn slightly to avoid obstacles, but you won't always have that option. Also, turning can be extremely costly in terms of performance.

Doing everything exactly right could make the difference between whether you clear the obstacle or not - and that difference could only be a few feet.

Best regards

[john_tullamarine]

aircraft,

In the olden days to which you refer, the OEMs didn't use the GAMA POH with its expansive text.

Vx is fine, if the aircraft is capable of climbing at that speed, the weather conditions are reasonable, the general terrain is benign, and the preflight sums have been done. Very difficult to generalise with Vx .. as it is with Vy. At some stage, however, in the continued takeoff, one needs to clean up and get to the cruise-climb environment. There is no easy answer .. which is why a lot of us prefer to abort if that is anything like a feasible option....

Consider that the POH words are there to meet regulatory and Industry convention requirements and protect the OEM to the maximum extent feasible ...

As much has to be read into the POH words between the lines as is read literally and, generally, the Industry pilot just doesn't have the background to read too far beyond the literal words.

Looking at some of the specific words you cite, it all depends ... for most twins at a decent weight, Vx is not all that relevant as the climb might well be negative ... after one cleans up (at whatever stage), one might be better off with around 2-3 degrees bank, primary controls rather than trim (provided one's manipulative skills were OK)

The POH intent is to control first with the primary controls and then trim when duties and workload permit .. certainly NOT an immediate priority .. depending on skill and currency levels, of course.

As you observe, "obstacle" is anything into which one might blunder ... even a gently sloping incline. The big problem, as I see it, is that much training is done at light weights and pilots get an unrealistic idea of what the beast might do when one quits. As for eyeballing obstacles, that is just plain imagination ... either one does the sums or one throws the dice on the day.

Turning flight involves a reduction in climb gradient ... no argument there ... it comes down to a case of which is better ... a straight flight path to higher obstacles or a turn, lose a bit of height, and head toward benign terrain ... again, something to be preplanned, not cobbled together during the escape procedure.

However, it is important that these issues be canvassed in a forum such as this so that different points of view can be argued. At day's end, it comes down to three things ..

(a) did we manage to get to a safe height and see a satisfactory conclusion to the flight ? If so, then a sigh of relief.

(b) if we aborted, did we walk away and can we argue the decision process in court ?

(c) if we continue and come to grief (hopefully walking away), can we argue the decision process and the flight management in court ?

Anyone who thinks that the arguing in court bit is not important should discuss the subject with some who have had to do just that. And, if you lose, it can be VERY expensive.