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adamant365

Flaps Question - High Altitude Landing

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Hello All,

 

I have a quick operational question regarding the 744 and flaps setting when arriving at a high altitude airport. I just performed Atlas 29 from KMIA to SEQM. Since SEQM is at roughly 7800' ASL, is it better to use flaps 30 to minimize ground speed, or flaps 25 for improved go-around performance? As it was, I was fairly light so I don't think performance would have been an issue either way. 

 

What if instead of 30 metric tons of cargo, I had, say 100 or more tons?

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Hello All,

 

I have a quick operational question regarding the 744 and flaps setting when arriving at a high altitude airport. I just performed Atlas 29 from KMIA to SEQM. Since SEQM is at roughly 7800' ASL, is it better to use flaps 30 to minimize ground speed, or flaps 25 for improved go-around performance? As it was, I was fairly light so I don't think performance would have been an issue either way. 

 

What if instead of 30 metric tons of cargo, I had, say 100 or more tons?

It depends on the situation, if SEQM was very busy and the chance of a go-around were high, then flaps 25 would be the way to go. If the airport was pretty quiet, then flaps 30 should be your choice. With 130 metric tons, I would go for flaps 30 in conditions where the wind was less than 10kts, and flaps 25 if there is a crosswind or winds ovver 10kts.

Hope I could help!

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I'm not sure it makes much difference. You should be at flap 20 almost immediately. GA mode will give you enough thrust for a 2000 fpm climb. If you were at flap 30 you will be slightly slower than at flap 25 and for the same vertical speed will have a better climb gradient. You could always select full GA thrust if this was not enough.

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According to my B747 performance tables, the landing climb limit weights are valid for both Flap 25 and Flap 30. In other words -- no difference. (Pax version, RR engines).

 

The aeroplane really performs very well and landing climb limit weight is not really a factor at all up to about 8000ft AMSL. Even then you still need temperatures in excess of about 30C in order for landing climb limit weight to be below MLW, and that's with all three packs on.

 

I would personally always use Flap 30 and the minimum possible autobrake setting when hot and high -- the B747 approach and landing speeds are fairly high as it is at typical weights, and if you are hot and high you don't really want to do anything that would increase energy level or increase the risk of rushing an already high-workload approach even more. Brake temperatures are more likely to be the issue.

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Flaps 30 would be my preferred setting, unless there are strong gusts or windshear. At a high altitude airport, a few knots of airspeed would increase groundspeed (and therefore landing distance) by a disproportionate amount.

 

A landing distance calculation (using TOPCAT) would be required with every landing.

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Make sure to get the SEQM freeware scenery from Carlyle sharpe in the avsim library here - David Lee

 

In fact I did download and install that scenery. It is quite nice :-). I'll be honest, I don't own any payware scenery. I'm more keeping frames high with my once "top" 2600K and GTX560. So, all I have for FSX is freeware sceneries. I much appreciate when someone takes the time to adjust land class, etc., as well as the afcad itself. Above and beyond for freeware. I'm usually quite happy with improved afcads alone (i.e. Ray Smith).

 

​Iain, Simon, Kevin: thanks for the input. I did end up using 30 to simply reduce my groundspeed and brake wear. I had assumed that the performance would be enough at flaps 30 in case of a go around, but wanted a bit more input. As it was, my VREF was only 133 which gave about 150 over the ground, calm wind. At a much heavier weight with VREF30 approaching 150 (VREF 25: 156), I'd be looking at close to GS 170 with flap 30 or close to 180 at 25. 10 knots at that speed makes a huge difference in energy to dissipate not to mention the amount of runway it uses up. Knowing that I can still use 30 even at high landing weight and have the performance for GA is nice. I forget how powerful those four engines are. Thanks!  

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Definitely Flaps 30, unless you have a lot of gusting wind at heavy weights, where flap load relief becomes a concern. The problem with hot and high is brake temps, as someone already mentioned. You also have to consider tire limit speed, which starts getting close at heavy weights and high elevations. I believe the tire limit speed is 225mph if I remember correctly. Don't know why it is in MPH oddly enough.

 

We land in Mexico city, which is at 7500ft, and I can tell you that on the -8, at max landing weight, you are right on the edge of the performance envelope. You almost always get the brake temp EICAS pulling up to the gate. Granted, the -8 MLW is 346t compared to the ERFs 302.

 

But still, thrust is not an issue on landing comparing flaps 25 to 30.

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