B777 RTA

[160knots]

Could somebody please explain why when ATC requires a time to cross a WPT, if that WPT and time is put in the Progress page, on many occasions it will come back with the message UNABLE. However it can still be achieved by manually manipulating the speed. So why can it be done manually but it cannot be achieved through the FMC?

The Boeing FCOM does not give a reason, it only describes how you can use the RTA function.

[777fly]

You are right that the FCOM does not shed much light on the RTA function. Based upon my own observations over many years, I offer the following:
The B777 has a 'supercritical' wing, which means that it is efficient within a limited range of mach numbers. This range is approx 0.815-855 M. Above and below these values the induced drag rises very rapidly. I believe that Boeing set the RTA values within this range to keep power required, and hence fuel burn, within sensible parameters and to also keep you away from high or low speed buffet margins. If your required RTA cannot be achieved within that speed range, you will get the 'unable RTA' message. Of course, you can set your own very high, or very low speed, to make good an ETA but you would be choosing to erode the buffet margins and substantially increase your fuel burn.
Standing by for technical explanations.......

[misd-agin]

.815-.855? You have anything to support those numbers?

LRC is typically .826

[nitpicker330]

What time tolerance does the FMC use to work it out?? +/-30 seconds??

If its less maybe there is your answer??

You only need to be within 30 seconds to keep ATC happy.:ok:

[zlin77]

On the RTA Prog page, bottom left corner it says Max .85....I assume a minimum speed will also equate to some specific value above the yellow line for buffet margins....which will vary considerably with altitude and weight..If I require a speed above .85, I just select speed intervention....87 is fun!!

[Sonny Hammond]

At our with most variants of the 777, LRC is always mid .83's. I've never seen LRC as .82 anything, maybe the company settings effect LRC.

[CharterJake]

@ misd-again
I haven't seen LRC to be anywhere near .825. My understanding is that LRC equals CI 140 which is about M.84. Company settings may influence this though I have difficulty seeing how. This is not initial buffer margin... We all fly the same wing. ( I know, I know rake on the -300ER/-200LR/-F vs. the classic wing)
Regarding the speed range of M.815-.855, I fully agree.
Unless you have access to the FPPM or complex computer flight planning applications the only easy way to support is playing with the FMC. On the low speed side: put a HOLDat PPOS and note the airspeed that it provides ( it will always be a CAS) and convert that into Mach. At moderate weights it will seldom provide anything less than .8-.815 at altitude. This will be very close to your minimum drag speed. On the high speed end just put .855 and .87 into the CRZ page, the fuel deficit is quite significant. Again I'm assuming moderate weights, not 351.5 T nor a 175T ferry.
My 2 cents worth.

[misd-agin]

My mistake. MRC is .826 LRC is higher.

We use cost indexes as low as 30. Highest I recall was about CI300. Difference in cruise mach is about .83 to .843.


@Charterjake - I engage ALT HOLD and speed intervention and enter other mach speeds into the FMC. It shows the time and fuel value of the changes. I'll play with the numbers again on my next trip.

I've yet to see a flight plan planned at .85. It's either .83 or .84 depending upon flight plan vs. schedule. Makes me think the company's decided the fuel cost of .85 isn't worth it.

[mach 84]

LRC always is about .84, LRC is not connected to any cost index and does not
care about any wind entered in the FMC. ECON CRZ takes the wind into account and will be around .83 when you get your planned/requested levels. Otherwise if you get stuck at a lower level it gradually would reduce speed to optimize your
fuel burn, however if you get your planned levels the difference between LRC and ECON will be about 5 to 7 min faster on a sector of 10 hours. If you select LRC over ECON, it will come at the expense of 100kg/ min you gain.

[Bpalmer]

I can't seem to find a reference for it in my 777 manual, but perhaps it parallels a function on the A330 which is similar.
That is when the speed is controlled by the FMC, the speed range is narrower than what the pilot is allowed to select.
For example, the FMC can only run the speed up to Vmo-10 while the pilot can manually select up to Vmo. Similar idea on the slow side, where the autothrust will limit speed selection to Greendot (flaps up maneuver speed-L/Dmax) but the pilot can select Vls.

[777fly]

misd-agin:
I don't have hard data on those figures, but I did the Boeing 777 course in Seattle back in 1995 and there was some discussion about the wing design and its most efficient speed range. There was a graph produced depicting the .815-.855 as the most efficient speed range and in my experience that is approximately the range over which the RTA function will work. Maybe someone at Boeing could come up with that graph again.

[misd-agin]

http://www.boeing.com/commercial/aeromagazine/articles/qtr_02_10/pdfs/AERO_FuelConsSeries.pdf

Page 7 - 777 MRC = .826 LRC = .840

Page 9 - Cost index chart for Boeing LRC speed. 777 = 180. 737-800NG = 35

Google. :D:ok:

[misd-agin]

Chart on page 7 shows the range for .815-.835 is fairly flat.

.855? Curve gets very steep above .84.

[777fly]

Misd-agin:
Thank you for the link to that interesting publication. My observation would be that the graph on p7 (fig 1) is a generalisation for all Boeing airplanes and may not represent the B777 exactly. In my company there was talk of raising the normal cruise to .85 due to the relatively low drag penalty, although that never happened. Whatever the actual figures are, RTA appears to be restricted to the .815-.855 range.

[misd-agin]

Head scratching moment trying to recall the RTA page, I think it's a two digit speed that displays.

I doubt the chart is a generalization. It has the correct numbers for the 777-200's MRC and LRC, and the nm/1000 of a 530,000 lbs(240K) at MRC(67nm/1000kg).

[maui]

160 K

Drop your CI back to zero and modify descent speed to minimum. This will give you your latest RTA.

It is my understanding,(can't reference it ATM), that RTA speed is a percentage of ECON speed, which will vary with CI, (which will, of course, have some relationship to LRC, and will vary with weight).

The biggest factor though is to get on to it ASAP, as often your RTA will be generated close in, with little time to achieve what you want.
Speed intervene to minimum acceptable, while you play star wars in the box.
When you have a resolution , go back to VNAV.

It is also worth remembering while you are doing all this fiddling, that the top of the yellow gives you 1.3 margin and freedom from buffet up to 45 degrees AOB, unless you are in turbulence.

Maui

[FullWings]

I remember experimenting with the RTA function when we first got the aircraft and found the speed went up and down a fair amount as small changes in wind, temperature, etc. fed their way back into the sums. OK if there is no speed control but if ATC are trying to maintain separation it gets them on your case fairly quickly as Mode-S tells all...

Overall, it seemed easier just to pick a speed and revise it occasionally, rather than having it hunt all over the place.