B737NG VNAV cruise Q

[Flight Detent]

Hi all,

Is someone in a position to tell me what controls the maximum bank angle at normal cruising speeds, at very high altitudes, in VNAV.

My understanding is that bank angle selection is only available in HDG SEL and VOR modes.

I'm curious since the minimum maneuvering and maximum speeds get much closer together in that environment.

Anyone...

Cheers...FD :uhoh:

[FlareArmed]

For LNAV operation:

-The FMC provides a real-time bank angle limiting function

-This function will protect the commanded bank angle from exceeding
the current available thrust limit.

-This bank angle protection is not available when LNAV mode is
deactivated.

Quote from a presentation by Catherine Davis of Boeing Flight Operations Engineering, entitled B737-High Altitude Maneuvering in May 2004.

[Capt Chambo]

Not exactly an answer to your question, but from my old copy of the FCTM, one of the criteria used by the FMC to calculate the Maximum Altitude, is the maneuver limited altitude.

"-the altitude at which a specific maneuver margin exists prior to buffet onset (minimum 0.2g for the FAA or 0.3g for the CAA/JAA operation). These margins provide 33° (FAA) or 40° (CAA/JAA) of bank angle protection to buffet."

(Reference the B737 FCTM dated October 31 2003, page 4.5)

[CaptainSandL]

FD, was VNAV a typo? I assume you mean LNAV.

The FMC route is constructed based on the maximum allowable bank angle (for the conditions, eg altitude) when in LNAV. The FMC software has been updated many times with various tweaks to the algorithms to prevent overbanks; the last was U10.6 where the nominal bank angle was set at half the track change but min 8deg & max 23deg. It had previously been max 25deg.

Note that this is only applicable when the aircraft is already on the magenta line, so when you engage LNAV or the A/P after T/O the aircraft may exceed this bank angle to get onto the track.

The actual logic of flying the track, or regaining it on a SID, comes from the FCC. Like the FMC this software is also updated, usually annually, and each version is improved slightly. You would think that after 40 years they would have got it right by now but new aircraft series/engines, winglets, features (Cat IIIb) and even autopilots (Collins) all require new software and some bedding in.

In short, you need to know what FMC and FCC software is installed and be on the magenta line to be able to accurately predict the bank angle.

S&L

[relax.jet]

I guess all guys here are correct but you better stay 2 levels bellow maximum initially, to see how bumpy it actually is. And only then climb to OPTIMUM. All the protections are there in LNAV, but as the angle of attack is changing in turbulent air you might be very quickly without any margin (especially in turns).

I have heard some rumours about -800’s with winglets to have some problems flying on top in bumpy air. (Optimum and maximum FL are little too close)

(My understanding is that bank angle selection is only available in HDG SEL and VOR modes.)

You are right about HDG SEL bank angle limit but it is not taken into consideration during LOC interception. So joining LOC it might go over.

Happy landings.

[Flight Detent]

Thanks Guys, I don't how I did that, of course I intended "LNAV"

Cheers...FD

[BOAC]

Airmanship of course allows you to select heading and reduced bank angle if you are operating in a 'tight' area and are concerned.

[stator vane]

what is that?

is that what they haven't spelled out into their ever expanding SOP's which attempt to standardize each thought, word and action---"Aviation for Idiots"

shouldn't it be Airpersonship anyway? ha.

that would make a good screening interview question!

"could you tell me what airmanship really means?"

would make more sense than the maths and pshyco questions they come up with.

[sire]

This may be a bit off topic but how close do you guys fly towards the amber footer up high?

I fly the -700 and when holding even above FL250 the FMC has you drone along at about 5-10 kts above the footer. Depending on how smooth the ride is I usually stay at least 10 kts above the footer and open the window if I have to.

The other day I flew with someone that was on the 767 for a long time and we held at FL 300 and above and I flew 15 kts above the footer and he was very uncomfortable about the speed. I did speed up some to make him happy.

So coming back to the thread, even at 410 and in heading mode with a 30 deg. bank angle, there seems to be sometimes plenty of speed margin towards the amber footer. Out airline manual has no guidance on the bank angle, yet most guys bank 10 or 15 degs only up high. Not sure if this is "technically" required though. I seem to remember a past thread on european carriers (Ryanair?) requiring to limit the bank angle at certain altitudes.

[plain-plane]

Just one (more) thing to remember: The FMC is preprogrammed with a very forward CG.
This gives you more protection, when it comes to the banks that LNAV will command. that is, unless you have changed the cruise CG.

[relax.jet]

Just one (more) thing to remember: The FMC is preprogrammed with a very forward CG. This gives you more protection, when it comes to the banks that LNAV will command. that is, unless you have changed the cruise CG. ??? What do you mean;unless you have changed the cruise CG ? Don`t you type in actual cruise CG on PERF INIT prior to each flight? We do ...

[BOAC]

R.J - it depends, as always, on company SOPs. BA USED to forbid it on the 737 (2004) - don't know now, and Astraeus procedures (737) do not include it.

Edit following mach79 - didn't notice R.J's error (RTFQ!):ugh: T'was cruise MAC entry to which I referred, of course. Who knows? Maybe R.J's company do actually calculate cruise MAC before departure, but as you rightly say, PerfInit requires MACTOM.

[relax.jet]

I know it's changing during the cruise (as fuel is burned off). But on PERF INIT page you are asked to enter cruise CG (CRZ CG) and on TAKEOFF page you are entering MACTO which is just for TRIM calculation; I guess.

If am I wrong, let me know pls. :bored:

(We are entering CG as expected on TOC.)

relax ;)

[BOAC]

No - you are quite correct - I really must stop trying to do several things at once.:ugh:

Cruise MAC is, as you say, entered on PerfInit and T/O MAC on Take off page. Neither BA nor Astraeus use the PerfInit setting.
I am writing out 100 times "Read the post properly":mad:

My apologies

[wee one]

Entering other than the fwd cg (5%) requires an approved proceedure not just random estimates of cruise cg/mac. If you dont have an approved proceedure for plotting the changes it should remain at 5.

Cross check the max man margin in the qrh and compare it to the max alt with the cg at 5% youll find then similar. Enter 20% and youll find a max alt much higher.
The qrh is thrust maneuver limited therefore bimbling along with you're new cg entered (probably because someone showed you and not because of proceedure) reuces considerably your ability to recover from upsets big wind changes etc..ie no spare thrust.

The UKcaa does not allow my company to do it and anyone silly enough to change the cg on a check would fail.

On a winglet aircraft the max and opt are much closer and the temptation to change higher Solution..get a proceedure authorised dont put a cg in because a skygod told you so. So back to airmanship not lookwhatiknowmanship or letstrythismanship. Another symptom of all singing and dancing jets. Older beings require a check of the qrh for the reasons above. the physics havent changed.

[intake]

When I was flying the 800 alot of the crew never had any idea about the crz cg! It would always default to 26% on power up. I found this a little strange as we often had crz cog's around 20-23%. Defaulting to 26% gave Max Alt's alot higher. If I recall some would set it correctly and some not. ......:bored:

[CaptainSandL]

Wee one

“Entering other than the fwd cg (5%) requires an approved procedure”The procedure is in the FCOM 11.40.26, it says “Displays default or manually entered cruise CG. Entry of actual cruise CG may increase maximum altitude.”

“youll find a max alt much higher”
It is only about 2-300ft higher in practice, not really "much higher".

“The UKcaa does not allow my company to do it”
The UK CAA allows many airlines to do it. Maybe your procedure was not to their liking? If the CAA really did not like it, they would have had Boeing remove the field.

The cruise CG is similar in value to the take-off CG that you will find on a loadsheet. One often used correction is to subtract 4 (figure will vary with series of a/c) from the MACTOW figure as that is the maximum possible CG shift due to fuel burn. On 1-500 series, cruise CG could be read directly off the trim wheel. I see no problem in entering the cruise CG into the FMC.

FYI the Boeing default, and FFA rules, CRZ CG is anywhere between 18 and 22%. The physics are no different on that side of the pond;)

[fivegreenlight]

CaptainSandL
I agree, on our 800's (UK ops) we subtract 5% from the mactow value and enter it as the cruise cg.:ok:

[relax.jet]

We do the same substract some value from actual takeoff CG. it was 4 for Classics and 5 for NGs. You can calculate the CG shift quite exactly. Use the trimsheet (manual loadsheet) in the same manner as you use it for calculation of takeoff CG. Just the fuel amount will be different....easy :ok: