Stall speed: how is it calculated?

[IefCooreman]

Hardy, yes. Boeing wants to show something to the pilots. Guess this is a discussion about "design philosophy", but this is only how I interpret what they are saying and how the system works and is supposed to be used.

Maneuver margin is indeed not about the top side only, hence the "line" starting from the stick shacker speed to top of the line, where Boeing says "full maneuvering margin". It's a "speed block" indication. (Flap maneuver speeds are indicated as a single short line, different style)

If the top of that line is calculated based on AOA and airspeed, the pilot does not have to worry about that. The bottom depends on g-load. What you get is a "maneuver margin" indication that can be translated as a bank angle availability. Top of the line is 25°+15° overshoot so 25° bank max for you as a pilot (you are not allowed to bank 40°...), bottom of the line is no maneuver/no bank availability.

This by the way is all "terminal maneuvering" because it is a flaps down terminal maneuvering aid, or flaps up in low altitude. In normal operations you use flap maneuvering speeds to have full maneuver capability (following Boeing normal procedures! We ain't testpilots...). In certain non-normal situations you can refer to the "maneuvering bar" to determine your maneuver capability. This is ie overweight landing or flap problems, where Boeing will also call for use of max bank 15° (for their Vref speeds as well).

[Will]

I'm having trouble understanding the difference between minimum maneuvering speed (top of the lower amber band) and flap maneuvering speed.

Minimum maneuvering speed is based on weight, angle of attack, and airspeed.

Flap maneuvering speed is based on weight.

For a given weight, as the AOA increases, the amber band "shortens" because the red band increases, but the green flap maneuvering speed should stay the same.

What I guess I don't get is this: Why isn't the top of the amber band always equal to the lowest visible green flap number? Put another way: why is the flap maneuvering speed usually greater than the minimum maneuvering speed?

[Hardy Heinlin]

When 4R on the APPROACH REF page is dashed, the flap speed markers vary with the GW indicated on the PERF INIT page in 1L.

When 4R on the APPROACH REF page is not dashed, the flap speed markers are based on the VREF speed in 4R and will no longer vary with the decreasing GW. *

The amber band is a caution range and will trigger certain alerts (company specific). If you were to put your flap speeds exactly on the amber tip without any safety margin, you would get lots of alert flip-flops.


|-|ardy


* E.g. when you enter 5/218 in 4R on the APPROACH REF page, the flap 5 speed marker will stay at 218 kt. If the pilot entered flap speed schedule were to vary with the GW, this function in 4R would be useless. You can enter any flap value before the slash. Normally, you enter 25/ or 30/, but these are not the only valid entries.

[IefCooreman]

Also, flap maneuver speed are designed to guarantee full maneuver capability during "speed transitions" when flap settings change: during normal acceleration after takeoff, or during normal deceleration on arrival. FCTM's usually have these nice graphs showing protection levels during acceleration and decelerations.

Minimum maneuver speeds don't protect you during flap configuration changes. But they give you guidance in non-normal situations where flap placard speeds might give you problems when using flap maneuvering speeds.

[Will]

Okay, that's helpful. Thanks Hardy and Ief.

[dhob]

When 4R on the APPROACH REF page is dashed, the flap speed markers vary with the GW indicated on the PERF INIT page in 1L.

When 4R on the APPROACH REF page is not dashed, the flap speed markers are based on the VREF speed in 4R and will no longer vary with the decreasing GW. *

The amber band is a caution range and will trigger certain alerts (company specific). If you were to put your flap speeds exactly on the amber tip without any safety margin, you would get lots of alert flip-flops.


|-|ardy


* E.g. when you enter 5/218 in 4R on the APPROACH REF page, the flap 5 speed marker will stay at 218 kt. If the pilot entered flap speed schedule were to vary with the GW, this function in 4R would be useless. You can enter any flap value before the slash. Normally, you enter 25/ or 30/, but these are not the only valid entries.

This is not accurate. The only valid flap entry prior to the slash is 25 or 30. If 5/218 were entered, then it would change to --/218.

Further, entries at 4R are for VREF only, whether FLAPS 30 VREF, FLAPS 25 VREF, or a Non-normal VREF. As such, whether 25/XXX is entered or 30/YYY is entered, the entire flap speed maneuver schedule is adjusted from the speed entered at 4R.

For example, if accomplishing the FLAPS DRIVE NNC, step 4 of the NNC states: "Use Flaps 25 and VREF 30+25 for landing." Assuming at max landing weight, VREF 30 speed is 156 knots. This adjusted speed is the new VREF and is entered as 25/181 at 4R. All flap maneuver speeds are now derived from the 181 Kt speed, as such Flaps 20 is 191, FL 10 201 kts, FL 5 221 kts, FL 1 241 kts, FL UP 261 kts.

[Hardy Heinlin]

This is not accurate.

What FMC version are you referring to?

The entries I described were valid on a legacy 744 FMC (Qantas) on the ground. Can't say it's inaccurate. Maybe it's different in the NG FMC?

Here's an example; "0/125" has been entered in 4R on the real deck for a test of valid entries:

[dhob]

This is for the NGFMC. I can't say if the Legacy allowed an entry of 5 or 0 before the slash. However, procedurally we never did. And the Non-normal procedure is the same, Legacy or NGFMC, and whatever VREF speed was entered, that speed adjusted the maneuver speed schedule for all flap speeds.

[Hardy Heinlin]

Have you tried entering something like 10/160 in the NG FMC? I understand that it's not a training procedure. I'm just asking as a software developer that needs to model both the NG and the legacy.

[dhob]

I can try some other combinations. I had just entered 5/Flaps 5 speed when flying a 747-400 w/NGFMC and it gave me --/XXX. Your post had me curious as I have never in 15 years on the 747-400 entered anything but 25 or 30 prior to the slash. I knew the speed schedule adjusted, but didn't know whether it would accept a number other than 25 or 30.