I have been following (as best I am able with the interruptions of the dreaded work) this interesting and informed topic centering around Cruise Levels , the FMC et cetera.

There is a lot I agree with and an equal amount that leaves me cold / puzzled. Rather than try and do all the little bits individually (since the topic has fragmented somewhat) I would like to put my "take" on some of the issues in one fell swoop (so to speak) and let you good people pull it apart from there. If nothing else this post might pull some focus back. Certainly I will take some "flak", but not a problem. That is the point of all this anyway is it not? If I could fly for another 35 years I might just get some of this stuff sorted out...

I must add my standard caveat about grandmother sucking eggs. Most of what follows is cut and paste from a series of write-ups I did a while back for some FOs (and a not a few Captains may I add) and I was trying to find foundation on common ground. Not always an easy task. So some of the expansions are a little slack on total accuracy. Give me a break on that.... I am working with a B777 on Trents and a Pegasus FMC - if that is not apparent. I am accused of having a light weight style on these things. But I offer no apologies - I find it works better this way.

---- Cut and Paste Stuff ------

Now here is an old friend NOT. Whoever wrote the explanation in the QRH is probably still in a mental home.

The Wind Altitude Trade Table (WTT) is one of the least understood and (consequently) least taught and used tables in the QRH. Which is a pity, since there is a wealth of information hidden in this simple table. However to understand the what's and why's of this, it is necessary to understand what the numbers mean / are and where they come from.

To start this at the beginning, we have to understand Specific Fuel Consumption (SFC). A simple definition would be Unit of Fuel per Unit of Air Distance (or the other way around - the numbers are easier depending upon the size of the beast). In this part of the world we talk, usually, of Kilograms per Nautical Mile. For a medium weight -200 at optimum cruising altitude it works out around 13.5 kgs/nm. The formula is simply FF/TAS. The important thing to remember is that the BEST SFC (lowest value) is, for a given weight, obtained at Optimum Altitude. ALL OTHER ALTITUDES will produce a WORSE (larger) SFC. If you understand that, then all of the rest is relatively easy. There will be those who want to say "But ISA?" Well ISA is largely cancelled out of this equation in a Mach Limited Cruise. More on that in a later missive. But remember SFC is a  STILL AIR  figure.

The numbers on the WTT are penalties, nautical miles in a sense, that you pay for being anywhere other than at optimum altitude. They are more specifically the number of Air Miles less that you will obtain from the amount of fuel burnt / Air miles travelled at one hour at the Optimum Altitude. So you can quickly see that where the table shows 0 is, in fact, the Optimum Altitude for that weight. All but the most mathematically challenged should now be able to understand the rather cryptic method that is printed below the table for using it as a WTT proper.

I would like to pass on the use of the WTT for its' original purpose by at the moment. We do not carry wind data for alternate levels (just now), so this is a rather inappropriate topic. However the WTT has much more to tell us.

Conventional wisdom and teaching has it that we fly 2000 ft on either side of (above/below) the Optimum Level (as shown on the FMC) for selection of cruise levels. As far as it goes, this rule works fine and it errs on the side of sensibility since the aircraft is always reducing weight as fuel is burnt. For those of us who do not know the Rule of Thumb: 1000 kgs off the AUW will increase Optimum (and all the other Altitudes) by 100ft. The B777 is not so conventional an aircraft though. What worked well for the 767 is not so good for the 777. Let's look at the WTT in some detail.

Of course there are lots of other factors here. The CFP levels, ATC, winds, weather. All these are important and each plays a part in altitude selection. However when you have a choice based solely on the numbers in the FMC give this a thought.

Which moves me, neatly, onto the next topic : Trim Drag.

Here's an explanation:

Trim Drag     When the tail of an airplane carries some load, several drag components are increased: the tail itself has vortex drag and lift-dependent viscous drag, but the lift of the wing must be changed to obtain a specified airplane CL: CLAirplane = CLAirplane + CLtail (Stail / Swing)

The increase in wing CL means that the wing vortex and lift-dependent viscous drag increases. In addition, wing compressibility drag is affected.

To compute this, we first must calculate the (negative) lift carried by the tail. For most transport aircraft without active controls this is about 5% of the airplane lift, but in the wrong (downward) direction. We could then compute the vortex drag of the combined wing/tail system and then add in viscous and compressibility increments. The difficulty with this is that unless we know the airplane center of gravity (CG) location, we cannot compute the tail load and in the early stages of the analysis, we do not know the airplane CG location. Sometimes we make rough estimates of the CG. When this is not possible, we can rely on more detailed computations done on other aircraft which show trim drag of about 1% to 2% of airplane drag. (I dragged this from my old AeroDy notes from CFS - they mean about the same now as they did then !!)

Well that's a mouthful, but it explains the thing after a fashion. In essence Trim Drag is a penalty that must be paid, in terms of increased Induced Drag (Lift Dependent) for the requirement to have the CofG forward of the CofP to provide dynamic pitch stability. Our modern B777 has everything going for it in this respect. The extra hold aft of the wing (to physically allow the CofG to be placed as far aft as required. An active Pitch Control system and an all moving tailplane help as well.. However Trim Drag is still there and must be lived with. But what is this to do with the price of fish? Well, quite a bit as it happens. Allow me to explain.

As far as I know, the FMC and the CFP both assume a CofG at 30%MAC and all performance calculations are based on that. If however the cruise CofG is away from 30%MAC then the induced drag penalties are more or less than assumed. Please note we are discussing CRUISE CofG here, not Take Off. (%MACTOW)

Now let me say something here before we get deeper into this. Firstly, nothing here is significant numbers on the safety side of things, that is definitely not the issue. We have a 5% fuel buffer that looks after all of this with total ease. Also, the numbers that I will be using below are pure guesses, intelligent guesses (I would like to believe) but still guesses based on (lots of) observation. So please treat what follows in the spirit in which it is offered. A guide, a thinking about point, maybe even something to examine further for yourself. On the shorter (<3 hours) sectors, none of this really makes any real difference. But longer sectors do show it all happening to a convincing degree.

Let me start by saying that our loading system is good and we mostly get a very good trim. Very good in the sense that it is better (more aft CofG) than the nominal 30% cruise MAC that is planned for. This is why we make a bit of fuel most of the time. However now and then it is worse. Then we lose a bit. But this "make a bit", "lose a bit" is a little inexact to my mind, and I would like to be able to get a feel for what is going on even if I have no control over it. Well in this case you can. The very simple rule of thumb here lies in the position of the Trim Indicator.

A cruise trim position of 4.7 seems to equate, almost perfectly, with the FMC/CFP figures. Each 0.1 unit shift represents an effective (apparent) change in cruise AUW (due to tailplane loading / Wing CL) of 1 ton. Trim figures less than 4.7 equate to Less Weight and figures more than 4.7 are equivalent to More Weight. And of course this is a moving target again. As fuel burns off, the CofG moves forward and then back again, and finally forward again (have a look at the manual load sheet). The higher you fly (and the lower the IAS/TAS) the trim moves back also. Fly too high and the trim starts to eat fuel - part of the reason for the WTT figures.

So now we can start to put some numbers to the penalty or bonus. Assume we start our cruise with a GOOD trim of 3.7 on the indicator. This is roughly a 10 ton weight / induced drag bonus over what the FMC and the CFP are working on. Look at the bottom of the CFP and it gives you a per ton adjustment for variations in AUW. Multiply this by your assumed trim bonus and then divide it by your flight hours. Over the next hour you should see this on your fuel checks, everything else being equal. And it really is there. If you work at eliminating all the other factors - it will show. The CFP is a remarkable tool and accurate fuel checks show all this happening. The converse is also true with a BAD trim, in the same magnitude. However do not assume that the TOC trim will be there for the whole flight. The trim does move as a function of fuel burn. A look at the EPR required figure on the VNAV (Cruise) page will also be a valuable clue. If you have a stable flight regime and stable EPR figures from the engines - compare those with the FMC predicted figures. If you want to look at the FMC's figures for your "apparent weight" as calculated from the Trim - go to the PERF page and (After writing down the existing ZFW and getting your other pilot "on-side" with what you are going to do,) change the ZFW up or down by the amount you "guesstimate". Now look again at the forecast EPR. Amazing stuff this rough and ready math... Don't forget to change the ZFW back to the PROPER figure when you are done.

OK, I agree, there is nothing that you can do about the trim of the aircraft. You are stuck with it. However this doesn't mean that you cannot be proactive about the effects on Cruise Altitude selection. This is the link and it is really quite important.

Think back to the beginning of this article were we saw the penalties for flying too high. Remember also that 1 ton of AUW was equal to 100 ft of Optimum Altitude (and Maximum Altitude but more on that later). The FMC does not know about your Cruise Trim / CofG - it assumes 30%, so all the altitudes are calculated to that. However an intelligent application of a suitable adjustment would give the wise pilot a definite edge. We are not talking big numbers here. It is rare to see a trim of 3.7 or 5.7, but in between is a whole world of variations. As with everything else we have discussed here, these are small effects with correspondingly small variations in performance. But in the long term they do add up either for the good or the bad. As I stated above, there is nothing here that is not generously catered for in the 5%, but on a redespatch day, sensible application of this knowledge can make that critical few 100kgs / hour difference sometimes.

You can't always beat the planned fuel, but it interesting to try.

My last word here is on Maximum Altitude. The bad old days of "coffin corner" are mostly behind us (on the B777 at least), but the beast is still alive, well and waiting for the unwary. Do be mindful of your trim when you venture to the top corners of the flight envelope for whatever reason. If you have a BAD trim then you could be working a perfectly acceptable performance margin (as far as the QRH / FMC) is concerned and actually have a pair of wings that are much closer to performance limits than you would knowingly want. This is a "how long is this piece of string" issue again and the margins that are built in are more than adequate, most of the time. The converse, of course, is also true, but that is definitely not an excuse to go higher than the FMC/QRH altitudes. But modifying the FMC's Optimum altitude with due regard to your "apparent" is an obvious bonus...

Enjoy...

I wanted to get into LRC / ECON and stuff - but this post is already to long. So I will leave it for another time. There is plenty of meat in the above for this topic to continue and I am looking forward to the feedback. Puts on Hard Hat and Kevlar Underwear ...

Ken B. Smith    (originally posted to BlueCoat 25/06/02)