B777 RECMD and OPT Alt

[extricate]

Hi there,

Question to all Boeing pilots,

Understand RECMD alt takes into account winds forecast whereas Opt Alt doesn't. What is your considerations to climb to the altitude under RECMD or OPT? Which one should we use in most cases?

[H44]

I believe for the 787 that Boeing considered doing away with Optimum as it was superfluous. We always use recommended on the 787 as like you say it considers winds so is a better level to use than optimum.

[Gillegan]

One thing to keep in mind regarding the REC ALT is that the FMC only looks at the winds for the next 500 nm. In some cases, when strong headwinds are forecast beyond 500 nm, the FMC will recommend a step climb when in fact, it's more economical to remain at your current level. I've only seen this on ULH flights. Because of this, it's a good idea to correlate any FMC recommended step climbs with the computerized flight plan.

[extricate]

I believe for the 787 that Boeing considered doing away with Optimum as it was superfluous. We always use recommended on the 787 as like you say it considers winds so is a better level to use than optimum.

Yeah I'm wondering why would Boeing provide Opt Alt? Serves to confuses more.

[haughtney1]

And...if you get REALLY bored on a ULR flight and the airspace allows it, pop 1000 into the step size and it’ll recommend steps in 100 ft increments, but only if you are super bored. Recommended is the number to use as long as the winds as accurate and you take account of lower as well as higher Alts. I’ve had wind info for 280 up to 400, but stayed at 260 for 2 hrs across Oz one night and saved 10 minutes and at least 1 tonne of gas thanks to getting Pireps of the actual winds...another thing to do if you are super bored.

[MarkerInbound]

One thing to keep in mind regarding the REC ALT is that the FMC only looks at the winds for the next 500 nm. In some cases, when strong headwinds are forecast beyond 500 nm, the FMC will recommend a step climb when in fact, it's more economical to remain at your current level. I've only seen this on ULH flights. Because of this, it's a good idea to correlate any FMC recommended step climbs with the computerized flight plan.

Recommend functions as if you are the only aircraft in the sky. I've seen it recommend a descent after hours of recommending a climb.

[wiggy]

After several iterations in procedures over the years we are now encouraged to consider the "step to" data rather than simply climbing at the behest of the "recommended" or "rec alt" figures, for various reasons.


FWIW according to our books "step to" does a cross over between the measured wind and forecast a few hundred miles ahead and at 500 no put has about a 90/10 bias in favour of the forecast, it doesn't completely ignore current measured wind even beyond the 500 mile point

On the subject, at least in part, I take it you folks are aware of this:

http://www.boeing.com/resources/boeingdotcom/commercial/services/assets/brochure/fs-windupdates.pdf

And the wind updates section of this:

AERO - InFlight Optimization Services Offers Airlines More Fuel-Efficient En-Route Operations (http://www.boeing.com/commercial/aeromagazine/articles/2011_q2/4/)

[JammedStab]

Usually, I go with the recommended. If the winds are stronger up ahead, one can always descend. In fact, the operational flight plan sometimes has us descend to go under a Jetstream and I believe that the Recommended will show a lower altitude as well based on its predictions.

Of course, most people try to have a buffer from MAX altitude prior to climbing.

I have noticed that usually the RCMD changes to a higher altitude prior to the actual green S/C point on the ND. Does anyone know why.

[wiggy]

I have noticed that usually the RCMD changes to a higher altitude prior to the actual green S/C point on the ND. Does anyone know why.

That's probably because there are subtle but important differences between how the FMC massages the numbers to come up with the S/C point verses RCMD alt figures. It would be interesting to see if you hit the green step point as the step distance reduces to zero........

There are documents around explaining this in detail but I'm wary of going any further 'cos I might be drifting into company/Boeing proprietary stuff - As perhaps you can see from the links I gave a couple of posts back Boeing have been doing a lot of work with operators regarding FMC use and bespoke (and paid for) provision of wind data.

[INNflight]

It would be interesting to see if you hit the green step point as the step distance reduces to zero........


Doesn't the FMC change from whatever milage it shows to S/C change to "Step to FL: NOW" once you cross the "S/C" dot on the ND?

That being said, we always use RCMD.
The only time we go by OPT is when engine out is activated during drift-down to consider the FL to descend to, but there it only shows MAX and OPT anyway.

[wiggy]

Doesn't the FMC change from whatever milage it shows to S/C change to "Step to FL: NOW" once you cross the "S/C" dot on the ND?


Yep point is (badly made by me) is that probably won't be the point in space where RCMD "steps" up. According to some sources it seems that the FMC processing of "step to" and RCMD is done slightly differently.

[extricate]

What's the relation between step-up climb and RECMD alt? Still in my early days of flying, a lot to learn still

[Willit Run]

I've done the step climb and the recommended , which were very close to the same, and as soon as I got to the new altitude, the box said i should decend back down.

So, after nine years , I climb when I have have a 2000' buffer between max and desired cruise altitude. I have also had the box tell me that recommended was the same as max. Not going there.........

And, this is on a 744 with the NG software. I will confidently stick to the older performance std's rather than the new data until they get it sorted out. With upwards of 60 known anomolies with the new software, me thinks they DON"T have it quite figured out yet.

[andmiz]

I have noticed that usually the RCMD changes to a higher altitude prior to the actual green S/C point on the ND. Does anyone know why.

RCMD looks forward up to a maximum of 500nm (250nm for 1000ft steps, 333nm for RVSM/ICAO/2000ft steps, 500nm for 4000ft steps) and recommends a level ONLY for that segment. STEP TO or S/C looks at the economy of the entire flight and will only suggest a step once it's 20% more efficient.
It's all in the manuals....

Highly recommend you follow the STEP TO unless ATC requests otherwise, then utilise RCMD as a preferred level, or MAX if you're forced up there (provided your speed won't change...maybe check LRC's MAX first).

[JammedStab]

There have been two things that I have been looking for prior to a climb. The first is that the new altitude will be 1000 feet below max(sometime 900). I admit that there is nothing that I have seen in any manuals about this and it just seems to be what most people do in order to have more buffer from the amber bands(any other recommendations are welcome).
Aside from considering clearance from maximum altitude, when it comes to when to climb, I choose the first of either S/C being indicated on the ND or RCMD showing the higher altitude. I do this because I figure that if I wait for both(which could be 15 minutes or so), someone else may have taken the altitude that I wanted and I am stuck below them for hours on end.

Once again, I welcome any input for improvement of technique.

[typhoonpilot]

The answer depends a lot on what specific type and model you are flying. For example some B777s are thrust limited and some are maneuver margin limited . The older B777-200, -200ER, and -300 are thrust limited airplanes and I used to teach not to go more than 2000 above optimum. The newer -300ER, 200F, and 200LR are maneuver margin limited.

Another set of factors is what is set in the airline policy and the CRZ CG in the FMS. The maneuver margin set in the older 777s is often 1.2G, while in the newer models it is set at 1.3G. CRZ CG in the older B777s is set at 30% while Boeing initially set 7.5% in the -300ER. All of these settings have a big impact on what the Maximum altitude will show and on the amber bands of the Airspeed tape. Understanding what they are set at and why is important prior to making absolute statements about how close you will come to the margin.

The B777-300ER is set very conservatively at 1.3G and (was) 7.5% (some operators now set higher numbers). That caused the Maximum altitude to often times show only 1500-1600 feet above optimum. Going back to the older models where Maximum Altitude was often 3500 feet above optimum shows quite a large difference.

Again, the older B777s were thrust limited so going more than 2000 feet above optimum would be risky if you ever got into any situation that caused a speed decay (turbulence, mountain wave, etc). On the newer models the reason given by Boeing for the more conservative settings was "an undesirable vibration" encountered in flight test. This was an airflow related vibration when maneuvering at altitudes greater than about 1500-1600 feet above optimum. Since the newer models are not thrust limited going to within 100-200 feet of Maximum is really a non-issue. It looks kind of scary on the Airspeed display because the amber bands are almost touching, but there really are zero performance related concerns operating that close to Maximum (and again it would only be 1400ish feet above Optimum).

Quite a few of the more senior captains at my old airline would often reset the CRZ CG on the B777-300ER to the old 30% or maybe even the actual CG in order to get a higher altitude sooner, especially when crossing India and the choice was either FL360 or FL280. These guys knew the reason for the conservative setting (there used to be a bulletin explaining it) and they also had a lot of experience operating the older B777s models so knew that going 1800-1900 above Optimum is pretty much a non-issue, especially when it meant getting out of turbulence or to avoid getting a very low cruising level as an alternative. Not saying that I recommend doing that, but it was known occur.