FMC altitudes

[GFCH]

On the VNAV page of the FMC used on the B767/757 on the bottom of the page it lists three altitudes OPT, MAX and RCMD.

Can someone shed some light on the logic used to arrive with theses numbers.

Also does the MAX altitude include anything for stall, g loading etc (IE.is it 1.3 or something else.

I've got my ideas but would like to hear someone elses.

I apologize for the lack of detail, just too tired to form a functional sentence but this is bugging me..

Many thanks

[Old Smokey]

Hi GFCH, welcome to PPRuNe!

I speak for the B777, which I believe to have the same FMC VNAV philosophy as the B757/B767. I'm sure that qualified B757/B767 will correct me if I am in error...:ok:

OPT : This is the Still Air (or zero wind) Optimum Altitude for your present Weight, and the Cost Index which you've entered. It is not permitted to exceed MAX. Useful if you have block levels and there's no significant Vertical Wind Gradient.

MAX : This is the most limiting of your Thrust Limited Altitude (Temperature), Maximum Permitted Altitude (AFM Limitations), or Buffet Protection Altitude (Usually 1.3 G as you've quoted, but some jurisdictions may allow it to be as low as 1.2 G).

RCMD : Recommended best level for your Cost Index entered, Step size entered, and Wind Component. With respect to the Wind component it IS very important to insert a reasonable range of winds above and below the Optimum Altitude such that the FMC may compute the best Wind/Altitude trade available.

Recommended is usually the level to fly at, but be aware that the FMC has no knowledge of actual or expected turbulence, and may push you all of the way up to MAX. MAX will only protect you for light turbulence, whereas you will need protection up to 1.5 G to 1.6 G in Moderate Turbulence conditions. For the B777 (non -300ER) this is approximately 1700 ft below MAX, get to know your own type characteristics of how much margin is desirable below MAX for anything more than Light Turbulence.

Hope that that helps, standing by for responses from real B757/B767 operators!:ok:

Regards,

Old Smokey

[LeadSled]

Old Smokey,
That's pretty close to the money. I am pleased to say that at least one well known operator selected 1.5 G, even 1.3 was cutting it fine, and 1.2 is just too touchy, with rumble in still air in moderate turns.
The theoretical fuel saving between 1.3 G and 1.5 G limits was never demonstrated in practice, in the real world.
Tootle pip!!

[GFCH]

Thank you gentlemen.

I had it pretty close. Flew with a fella the other day who thought MAX was the max altitude the a/c could fly at period and wouldn't go within 4000 feet of it.

I couldn't convince that protection was included in that altitude.

Cheers

G

[Old Smokey]

GFCH,

You may assure the fella who wants to avoid MAX by 4000 feet that there IS protection there, to varying degrees as discussed.

You may also assure him that if given the choice of flying with him or the turkeys who want to fly at MAX, I'll fly with HIM any day.

A tad conservative, but conservative people usually live longer....:ok:

(BTW, 4000 ft below is approximately right for SEVERE Turbulence), he does have a point............. I'll bet he flew in the old pre-FMC days when that was the rule.........

Regards,

Old Smokey

[PappyJ]

A tad conservative, but conservative people usually live longer.... So very true!

Always a pleasure reading the literary works of Old Smokey...:ok: