DRAG REQUIRED: How often?

[Will]

Just wondering how often the DRAG REQUIRED message appears in real-world 747 operations? Is it something you see on nearly every descent? Or is it rare?

Similar-but-not-exactly-the-same question: How often are speed brakes used in real-world descents? Just about every flight? Or rarely? Or somewhere in between?

[Avi]

You should know better than us, you were there 

As far as I can remember speed brakes were used in every flight I had. Even at night flights I could feel the aircraft "vibrate" when they were activated.

You will get the Drag Required message only if you above the path and the extra speed (in VNAV PATH) is not enough to close on the glide path. Normally, I think, ATC will tell you to descend even before TOD so you very rarely should find yourself above the glide path.

If it happens to you in PSX too many times, are you entering the wind data into the DES FORCAST page (for best TOD calculations)?

Cheers,

[Hardy Heinlin]

I remember we once got very steep descents on the arrival into Kai Tak with full speedbrakes, starting from cruise level. That was a 747-400; I didn't hear any vibrations.


|-|ardy

[Britjet]

I remember we once got very steep descents on the arrival into Kai Tak with full speedbrakes, starting from cruise level. That was a 747-400; I didn't hear any vibrations.
|-|ardy

That happens into the new airport as well. The promulgated STAR from Europe is very drawn out - a long way south over the sea - but they will sometimes "snatch" you for a straight-in to 07 (or 13 as it used to be). It was often a good idea to brief the cabin crew that the arrival could be 10-15 minutes earlier than the FMC indicated in cruise.
Peter

[Will]

You should know better than us, you were there.

I was nearby. Not exactly there, but nearby. :-)

(I've yet to ever be in a 747 cockpit.)

Let's say that ATC is minimally restrictive, you have the most recent available enroute winds and descent forecast winds loaded into the FMC. In what percentage of descents does the aircraft find itself so high above the VNAV-calculated glide path that the DRAG REQUIRED message appears?

[United744]

You should know better than us, you were there.

I was nearby. Not exactly there, but nearby. :-)

(I've yet to ever be in a 747 cockpit.)

Let's say that ATC is minimally restrictive, you have the most recent available enroute winds and descent forecast winds loaded into the FMC. In what percentage of descents does the aircraft find itself so high above the VNAV-calculated glide path that the DRAG REQUIRED message appears?

Assuming Boeing did a reasonable job with the performance calcs, never?

The only time (really) this would happen is if the forecast/entered winds were different to actual, causing the computations to be wrong (generally this happens if you forecast higher headwinds or lower tailwinds than actual, or lower OAT than actual - higher OAT results in higher TAS vs. Mach).

[Hardy Heinlin]

A large baro pressure change within a short leg that contains a TL/TA transition point may also affect the original plan.

It's also important to have the correct TRANS LEVEL entered on the DES FORECAST page. This may change an altitude constraint by a few hundred feet in the worst case; it makes a difference if you focus FL120 with QNH or 12300 with STD.

Also anti-ice usage (high idle thrust setting) that disagrees with the TAI ALT entry on the DES FORECAST page may have an effect on long idle descent paths.


|-|

[Magoo]

The game is not to touch the speed brake of course, as for the drag required, if you're familiar with the airport and fine tune the FMC's legs page with what you know will happen you'll never really get to see this message.