B737 Questions

[B777Heavy]

Hi,

I got 2 questions regarding the B737

1- generally on the B737 pressurization panel DCPCS, what do you put on the LAND ELEVATION window? Do you dial in RWY ELEVATION or FIELD ELEVATION? The reason I ask is there is quite a disparity between the 2 in airports such as Harare and Cairo where the difference is 200' or more. The FCOM says FE however, doesn't that beat the purpose of the land elevation that lands slightly pressurized to minimise effects of the pressure bump in airports such as FVHA?

2- specifically on the B738, quickly a poll, save for the FCTM conditions, would you generally prefer a F40 landing or a F30 landing and why?

[Capt Chambo]

1. Our manual says field elevation, but frankly don't think it really makes any difference. (There is a special case when landing at airfields whose elevation is > 6000' see the Supplementary Procedures)

2. Assuming that you have already consulted your normal landing distances Vs Flap Vs brake settings, then my general rule of thumb is if my Vapp (i.e. Vref+additives) is going to be 150kts or more using Flap 30 then I use Flap 40. However if turbulence or windshear is forecast or is likely, or reported, then I stay with Flap 30 and wind the brakes up accordingly.
The reason why: I used to fly for an outfit that had the BTMU option. It seemed to me that we often parked with hot brakes, after landing Flap 30, Vref>150Kts, AB3 and/or "stomping" on the brakes to make turn offs. Whereas Flap 40, AB 2, Vref <150kts rarely resulted in hot brakes. Just my observations over the years!

[de facto]

It seemed to me that we often parked with hot brakes, after landing Flap 30, Vref>150Kts, AB3 and/or "stomping" on the brakes to make turn offs. Whereas Flap 40, AB 2, Vref <150kts rarely resulted in hot brakes. Just my observations over the years!

And yet, a single check every flight (not years of trial)in the PI section of your QRH under 'brake cooling section' would have given you the answer whether you were close to having hot brakes/long turn around time and if indeed a Flaps 40 and /or maybe AB 2 was a better solution.

[paperdragon]

Dont know matie, how many years of flying that remark is based upon...rule of thumb is in my book always easier than getting the QRH out :ok:

[FlightPathOBN]

I have noted many airports where hitting the deck and stomping on the brakes is tearing up the asphalt on the runways as well....

[root]

Dont know matie, how many years of flying that remark is based upon...rule of thumb is in my book always easier than getting the QRH out :ok:

Rule of thumb is also more often wrong than the QRH.

[HAWK21M]

QRH is for a purpose...Use it.

[captplaystation]

Common sense would dictate that a field with a large difference in threshold elevations would lead one to set that in preference to field elevation, but. . . some companies discourage the use of common sense.
Can't help thinking that when Boeing wrote the manuals all those years ago, they reckoned that approximation was OK, if they ever gave it another thought, they probably came to the conclusion that the system wasn't so precise anyhow & left it at that ,as it had never caused any problems.


As the 2nd Q asked "preference" rather than procedure, well, easy one . . 40 every day.
Why ? aircraft is more speed-stable/less pitch sensitive on approach, if heavy, speeds are more reasonable, much easier to put the aircraft exactly where you want it on landing as the flare does not demand the level of absolute precision demanded by a Flap 30 arrival, so, less chance to either underdo it (crunch ! ) or overdo it (floating along) than F30.
As to crosswinds, well I have done enough x-wind landings on RW's short enough to encourage F40 (even if F30 was theoretically possible) to say, I never found the handling wanting with F40, & if gusty, much prefered the ability to put the damn thing on the ground,exactly where I wanted, rather than flare /float/drift/Ooops which I have seen a few times on 738 in gusty conditions, by my own hand & particularly by inexperienced new FO's.

Only 5000hr on 800, maybe I haven't hacked it yet :rolleyes:

[Denti]

The company encourages flaps 30 landings to safe fuel. However with any tailwind component flaps 40 is mandatory. Autoland can be done in either flaps 30 or 40, however flaps 40 is recommended except for OEI approaches where it is prohibited for performance reasons (go-around climb). Personally i rather use flaps 40 on the 800 and mostly 30 on the 700. But it depends on the runway, desired turn off and weight and therefore Vref, don't like Vrefs much about 145kts. The 700 is much slower which i prefer, except in bumpy conditions.

[root]

As the 2nd Q asked "preference" rather than procedure, well, easy one . . 40 every day.
Why ? aircraft is more speed-stable/less pitch sensitive on approach, if heavy, speeds are more reasonable, much easier to put the aircraft exactly where you want it on landing as the flare does not demand the level of absolute precision demanded by a Flap 30 arrival, so, less chance to either underdo it (crunch ! ) or overdo it (floating along) than F30.
As to crosswinds, well I have done enough x-wind landings on RW's short enough to encourage F40 (even if F30 was theoretically possible) to say, I never found the handling wanting with F40, & if gusty, much prefered the ability to put the damn thing on the ground,exactly where I wanted, rather than flare /float/drift/Ooops which I have seen a few times on 738 in gusty conditions, by my own hand & particularly by inexperienced new FO's.

Only 5000hr on 800, maybe I haven't hacked it yet :rolleyes:

I find the aircraft becomes rather unstable in roll with F40 at low speeds. I experience fewer wing drops during the flare with F30 as opposed to with F40. However the aircraft does settle easier with F40 and doesn't float nearly as much.

Both F30 and F40 have their merits.

[captplaystation]

Indeed, on a related subject, does anyone have an explanation for the slightly worrying "buffet" felt when rolling the wings level at (say) 300' on a circling approach with aircraft stable in power/descent rate with F40 ?
First few times I felt it , 10 years ago, I thought "Sh1t shouldn't the stick shaker have saved me from this" it is a very odd sensation.
Something aerodynamic no doubt, a bit like the buffeting on other types with full flap, but only happens when you are actually rolling.

[M.82]

About your question number 1, I found this:

"In the AUTO or ALTN mode, the pressurization control panel is used to preset two altitudes into the auto controllers:
•FLT ALT (flight or cruise altitude).
•LAND ALT (landing or destination airport altitude)."

FCOM B737NG, page 2.40.4

I also know that the Pressurization controller set the cabin altitude 200 Ft below when you set TAKE OFF THRUST, and also 300 Ft below when you land, but I couldn't remember where I read it.

In my company, we set airport altitude.

[B777Heavy]

@captplaystation. Makes sense F40 does feel so much more stable.

[B777Heavy]

Hmmm. Ur speeds were at Vapp? And u still had the buffet?? Haven't done circling on 800.