Anti-ice perf correction on TO

[Justbelowcap]

Can any of the Perf A whizz kids shed any light on when I should make a correction for Eng Anti-Ice selection.

Should this only be applied if the starting conditions require it's use or should it be applied if any part of the initial climb requires it's selection?

It would be really useful to have references which clearly state it should only be the surface conditions..or not.

Many thanks.

[Captain Stable]

Are you asking for a Perf A exam to get the mark, or asking what happens in real life? The two differ quite widely.

[Justbelowcap]

Not sure why the answer should be different?

For more years than I care to mention I've only been applying the anti-ice perf correction if the conditions warranted at the surface. i:e I've not bothered to take any account of cloud below the single engine profile. This was the way it was always taught on the various aircraft that I've flown. However one of my companies keener training FO's is now making it company policy to take into account any use of ice in the initial climb out. I accept the logic but wonder why this has never been pointed out before. I've an inkling that it's only the surface conditions that need to be taken into account, as in the wind case. It would be great if somebody could point in the direction of a "official" ref that could clarify the issue.

[BOAC]

I cannot give you the reference - I'm sure someone else can, but I think your TFO is correct. Think of it this way - your RTOW is calculated to ensure not only getting off/stopping on the runway, but also to clear ALL obstacles in the climb-out phase. Engine thrust is (normally) reduced with anti-ice so this is valid up to the end of the acceleration phase.

[Empty Cruise]

Justbelowcap,

I'd tend to agree with your trainer. If icing conditions are present below acceleration altitude, e.g. OAT on ground 4 deg. C, clouds bkn 200 ft., you would (in most aircraft - check your AFM/FCOM) depart with ice protection switched ON.

...or you might have a failure at V1, and then have to both fly the aircraft, complete memory items, select anti-ice on and bug the new speeds - all below acceleration altitude - doesn't sound like the fastest way to a long and happy life

So yes, the anti-ice should be ON (again, AFM depending...) and the appropriate performnce penalties and speeds should be applied.

...eeerrr, methinks...

Brgds,
Empty

[Justbelowcap]

OK I accept the logic... but why then do we just accept the surface wind conditions? The wind has a far, far greater effect than anti-ice but we don't have to consider wind effects above the surface. At what height then do you decide that the cloud no longer has any effect? You certainly wouldn't select anti-ice on before TO if there was cloud at 1000ft (or 1500ft or 2000ft), that would be selected in the climb, but the debate in my company is whether the ice correction should be applied if you feel that ice would be selected at some stage during the climb out.
(Obviously if you elect to use anti-ice on the TO roll then you would have to make a perf correction but nobody is going to pre select anti-ice for a dry RW because there is cloud at 1000ft)

Would love to find a ref that confirms it either way.

[BOAC]

OK - wind: Generally conforms to a well-known model with height and is, I guess, 'allowed for' in the performance. You could have thrown-in a temperature inversion. We cannot allow for ALL possible deviations, but icing is fairly easy to determine?

My point again - your 'guaranteed' engine-out performance is required up to 'clean', so include the anti-ice in the calcs. because that affects your max RTOW. When/where/if you turn it on does not matter.

[Justbelowcap]

We do allow for temp inversion in so far as we aren't allowed to use a reduced TO power setting if there is an inversion.

Just seems weird to me that for years and years nobody has mentioned that you have to take into account low cloud in a Perf calc. None of the worked examples given in our manuals makes any reference to this factor and I would have thought this would have been a classic gotcha and highlighted over the years.

Does the Perf calc end when the aircraft is climbing away clean?

[Empty Cruise]

Lats say you had icing in final segment, but not in second segment.

In this case, you would come up with 2 limiting masses, 1 for 2nd segm. and 1 for final segm. (don't know how your RPM lokks, but something like that).

Wouldn't the lower of these two mases simply be your MTOM vs. OBST? Methinks, but maybe your perf. documentation does not differentiate.

Brgds,
Empty

[4dogs]

Folks,

Our normal take-off planning is based on worst case, ie an engine failure at Vef. If the engine fails later, we are much better off but have no way of assessing just how well off - worst case planning remains our best protection.

If it appears likely that anti-ice will be required before the take-off is complete, then worst case planning would naturally lead to calculating the take-off as an anti-ice ON take-off, regardless of when you actually select it.

The take-off is complete when the flaps are up and climb power is set, usually above 1500' AAL or higher if terrain so dictates.

Stay Alive,

[Justbelowcap]

Again I totally accept the logic BUT where is it written down?

ATIS
27R 270/10 10km ovc011 9/5 1023

Would you have made an anti-ice perf correction? I wouldn't have for the last couple of decades. I would have thought this would have come out in at some stage in the past. Perhaps I've always missed it but no FO I've flown with has ever made an Anti-ice correction in the above corrections.

[BOAC]

I'm eagerly awaiting JT here to answer this, but ITMT, check JAR which will define the 4 (poss 5) stages of 'take-off' - normally up to 1500'AAL.

Consider now the fact that e/out perf gives you a ?35'? (not sure, but not much!!) clearance on any obstacle in the flight path and ask yourself how much loss of height you will get with Eng a/ice on and is it crucial?

In the case you quote you will probably be using a/ice from around 1000' when you decide to put it on until clear of cloud, but at least until 1500'AGL unless the cloud is very thin. If I assume your example is LHR 27R, then I would suggest (and wait to be pilloried for it!) that it is not that crucial (unless the standard is flying on Windsor Castle)? Why not ask your Chief trainer?

[4dogs]

Justbelowcap,

Is it specifically written anywhere? Maybe, maybe not - a regulator wouldn't go near it, simply because it is a function of each aircraft's systems and their associated limitations and operating procedures. And they don't have to because they already have a rule that says comply with the AFM!

Let's say my AFM requires anti-ice to be selected on 2 minutes before entering icing conditions - if I am intending to comply, then it would generally take me less than 2 minutes to climb AEO to your 1100' icing level, so I should be prepared to turn the system on just after lift-off or, more practically turn it on to start with.

What we generally do is calculate both figures and make an informed choice when we can see our take-off path.

If there is no time limitation, then generally my reduced take-off thrust with anti-ice OFF will provide me with adequate EGT/TGT margin to select the systems ON as required.

Our approach to the issue is simple: since it is more of an airmanship issue than anything else, we consider it to be part of the commander's operational discretion to ensure that the aircraft has the appropriate anti-ice protections in place before entering icing conditions. Moreover, our FOs will ensure that the commanders meet those requirements.

Stay Alive,

[john_tullamarine]

Interesting thread with no simple answer, although previous posts have covered the main considerations ...

Some thoughts as I read through the posts ..

(a) certification data don't give you any guarantees .. rather the AFM suggests that, if you do certain things under certain conditions, then there might be a reasonable chance that the observed performance will be such and such ..... of course, at the enquiry, the bewigged ones will probably take a diametrically opposed point of view ...

(b) temperature and wind profiles, humidity and just where what devilry is going to happen to you, are all a bit hard to predict with great precision at the time you advance the throttles/power levers/whatever

(c) the operating company ought to publish sufficient guidance as to the bases of scheduled performance data so that crews can make intelligent decisions (beyond SOP considerations) as to how such data might best be applied in the real world

(d) the accountants might not want to see you err too much on the side of conservatism .. while your wife/husband/partner and the people down the back might prefer a more risk averse approach to life

(e) whatever you do, it is comforting to have a defensible position to argue at the enquiry, court case, whatever. It is definitely not a good thing to have opposing counsel ask a question which leaves you with only a facial blank look of horror response option ...

(f) wind is addressed for the climb out ... certainly in all the AFMs I have worked with ... usually by the performance engineer nominating the wind to go into the calculation (by one means or another) ..... just how closely the presumed wind in the calculation might compare with the real world wind is another consideration altogether ... clearly if you know that there is a significant low level wind shear, it is a bit silly to blast off, having ignored that observation, with brain in neutral

(g) as an aside, for near surface wind with height variation, the usual presumed profile is

W1/W2 = (H1/H2)^(1/7)

This is reasonably representative of typical real world conditions in a steady wind field and in the absence of ground obstructions. It is the usual formula built into your AFM charts ... however, I wouldn't use it much above 100 - 200 ft or so

(h) the 35 ft clearance bit relates to a calculated basis for the RTOW (sorry, RTOM for the purists).

The certification test data are fudged a bit to give the AFM "gross performance" figures which account for a reasonable variation amongst individual aircraft. The gross data are then fudged a bit further to give "net performance" figures which are used in keeping the calculated net flight path above the hard bits sticking out of the ground. The calculations made by the performance guys and gals are predicated on a minimum of 35 ft clearance between the terrain/obstacle profile and the calculated net flight path.

In the real world, normally you don't have an engine failure and the aircraft goes like a cut cat ... with twins doing better than tri-motors which, in turn, do better than big birds. These latter observations are a consequence of the physics and the gross to net margin requirements.

If you do have a failure your performance, under any half reasonable conditions, probably will be at, or a little better than gross performance expectations.

When you consider the gross to net decrement, this becomes very significant to the pilot in the early stages of the takeoff because the decrement hasn't had enough distance from end TODR to give you much in the way of fat. As the aircraft progresses through the segments, the decrement progressively gives you an increasing height delta between gross and net.

(i) as to whether the V1 failure scenario is critical depends very much on the planned flight path. If the normal departure is the same as the escape path, then fine. If, however, the two are different, then it is necessary to consider failures at all relevant points along the normal flight path until a safe height has been achieved ... I know that some companies DO NOT do this and ONLY consider the V1 failure case.

... and, as Richard 4Dogs observes above ... at the end of the day it is not feasible to prescribe away all the worries by having ever bigger Operations Manuals ... sometimes the guy or gal in the left seat gets to earn his or her salary. The old saying remains ever true ... the captain doesn't get paid for having a routine flight .. rather he/she earns the salary every (infrequent) once in a while.

[mutt]

At least one of our Douglas aircraft has an "AUTO" setting for Anti-ice. Takeoff weights are therefore produced with Anti-ice ON for temps below 8°C.

BOAC, we operate in an environment where summer temperature inversions of +15°C are common. We actually plan the takeoff weight at the inversion temperature, beancounters be damned....

J_T.

If, however, the two are different, then it is necessary to consider failures at all relevant points along the normal flight path until a safe height has been achieved .

Is that really feasible? Boeing has only just produced the BCOP program for all engine departures. Prior to this and with other aircraft, a task such as the one you described would have to be done manually. We design engine failure procedures when we want to increase takeoff weight, normally this would involve turning away from a limiting obstacle, while the SID may bring the aircraft straight for that obstacle. If we were to start accounting for the worst case scenario for that SID, we would have to consider it limiting and ignore the benefits of the engine failure procedure.

Mutt.


Off to the land of Guinness, so advance apologies if i dont get to post on this topic again.

[john_tullamarine]

Mutt, my good friend, g'day ... I knew it wouldn't take long to get a bite on this point.

Short answer, as you well know, is ... "it depends"

(a) if the AEO and OEI paths are the same .. no worries

(b) if the OEI turns away from a reasonably distant obstacle and the rest of the area is benign ... usually no worries

(c) consider, though, a case where the takeoff is from within a valley with only one feasible OEI escape path through a saddle which can only be achieved by a turn shortly after takeoff. The AEO departure is over tiger country. V1 cut .. no worries ... track via the escape. But what about if the failure occurs AFTER the turn point for the V1 plan ?

(i) do we just say ... " tough luck" and order a replacement hull ?

(ii) do we just say ... "problem belong pilot", call the company's defence legal team, and prepare to hang the pilot out to dry ?

(iii) or should we have thought about that late failure case ? or perhaps have scheduled ALL takeoffs via the OEI escape track ?

Many runways fit into the spectrum between (b) and (c) ...

As to actually doing the sums, I have never had any major problem in setting up a reasonably simple PC program to do the analysis ... monitor the presumed AEO performance and then consider the options in the event of a failure. Certainly not a five minute exercise but, nonetheless, not technically difficult. If the AFM has not gone through the regression exercise and you don't have an OEM program ... it can get a bit frustrating ...

... as a for instance ... I can recall one runway where the options were

(a) takeoff over the saddle located some distance off the runway end and wear the payload losses

(b) schedule a turn at runway head

I spent days doing the thing manually looking for a VMC departure which got around the OEI losses involved in the straight ahead takeoff (aircraft MTOW permitted a reduction in splay width in the particular country) ... eventually I had to admit defeat and just run with the straight flight path.

.. as another for instance ... I have done a number of schedules from a medium elevation airport surrounded by lots of bumpy bits .. in this case, the critical failure for one of the main runways was very definitely some distance out ... the V1 cut was not very relevant for continued flight path obstacle clearance.

But the point is ... is it acceptable just to run the V1 failure analysis and merely presume that the AEO track is immune from OEI problems ? ... without at least having a think about the thing on a runway by runway basis ?

[4dogs]

Mutt,

and I hope the Guiness is to your liking, to be sure....

As is often the case, I find myself in very heated agreement with JT on expanding our horizons beyond the failure at Vef. In most of my recent analyses, that is the easiest case because all of the complicating factors are known to us and we can plan with pretty good data.

But in our continuing search for excellence in providing performance advice for flight crews, the failure at Vef is only the first step. The very next step is to look at what happens if the engine fails down-track after the aircraft has left the Vef/special departure procedure flight path. That may result in a more hazardous terrain scenario than the Vef failure case.

As you correctly identify, getting useful AEO data to supplement the OEI data is a major problem. I am sure that all of the data is available from the OEMs but they don't crunch the numbers because it is not AFM prescribed data. In several cases, I have had to resort to using the simulator to create some very conservative worst case data to provide a starting point for the "what-ifs".

You are also quite right in that the "what-ifs" become a Pandora's Box of possible flight path-failure point calculations.

However, my experience in producing RTOWs (mass, I know I know!!) for OEI SID compliance shows a substantial commercial penalty that I cannot justify. The other thing of note is that SIDs are designed purely for the benefit of ATC and the airways structure - in my part of the world there is no real operational analysis and the ATS provider serves no one but itself. I try to look for the critical point on the SID where net performance will provide for the relevant obstacle clearance (ie 35' plus the gross-to-net distance benefit).

I don't have the resources to redo all of our data, so we proceed on a risk management basis as best we can. The bean-counters only see us as an impediment and ignorant flight crew don't often help with the cause. However, the challenge that is emerging is in constructing simple solutions that are flyable without relying on technology or the PNF chattering away like a car rally navigator when there are many competing priorities.

The good thing is - we are getting better at it and we are getting excellent co-operation with the flight crews in designing better operating procedures to complement or to compensate for our performance planning limitations.

The bottom line remains that I want as complete a set of contingency options available to me every time I take my feet off the brakes.......

Stay Alive,

[Empty Cruise]

With fear of leading this thread further away from the original subject...

...do you brief intended flightpath in case of failure after the point where normal climbout track diverges from OEI-track? As pointed out previously, there exist no calculations for this scenario...

I must admit to briefing so at certain airports, albeit without any kind of legal back-up or prior calculations that can prove that this is the right way to go. However, if I have started a turn toward rising terrain on an SID and I suffer a flame-out, I will NOT roll out on a heading taking me toward mountains (SID saying so or not), almost no matter how high above the Vef engine failure track the failure occurs. My intention would be to continue/reverse the turn toward lower terrain or a HP that I could climb in - an I would brief so prior departure.

J.T. - good post as usual! But on a personal note, I wouldn't rely on the 0,8% point 2 eng. gross-net diff. to take me out of any mess that may lurk up ahead, be it ice or terrain. The manufacturers do everyting legal to keep gross performance as good as possible - it sells airplanes. Recently read an article in a Businnes Jet magazine about the new Cessna 650XLS - where a substantial increase in landing data performance was obtained over the 650, based - not on new brakes, not on new anti-skid system, not on new spoilers - but on improved certification test procedures

Ah, now I feel much more happy taking on that 1200 m. runway

Brgds,
Empty

[john_tullamarine]

Looking for worst case options need not produce an overly complex escape brief .. and, if it does, then we need to go back to square one and rethink it.

Point I am trying to get across is that the V1 fail scenario does not necessarily address the worst case.

.. and winging it with hands clasped in supplication is not much of a strategy to defend in court.