Hey guys. I

In reality, we use pre-calculated performance charts; one for each airport given that every airport will have its own altitude and weather conditions (i.e. temperature, pressure, wind).Airlines, depending on their area of operation, favour towards different set of flaps given that it adopts better to their type of flight.Now, a reason for GOL to prohibit flaps 1 T/O as opposite to Continental might be that GOL, operating in Brazil, has to face with hotter temperatures, this goes against aircraft performance.So, how do we choose takeoff flaps? Depending on aircraft weight, runway length and obstacles and the weather conditions stated above (to name a few). For example, if there's an obstacle right ahead, you choose a greater set of flaps because you need to clear the obstacle.In normal scheduled operations (i.e. airlines), most of the times you use one set of flaps, which is the one the carrier has deemed to be more favourable to their operation and unless special conditions are present (like a specific broken part of the aircraft, max takeoff weights at high airports ), you will use that set of flaps.For example, in the MD-80, you most likely will use flaps 11. On Mr Boeing airplanes you are likely to use flaps 5 (considering the factors stated above).It doesn't have anything to do with initial climb fuel efficiency as the segments are short to make a significant difference. Add that to the fact that you reduce to climb thrust no higher that 1000ft on any aircraft.Hope I made myself clear.Best regards

Hey guys. I

Bruno and Ed, thank you for your replies. I had already ran a search in the PMDG forums but got no hits. Perhaps I didn

Lacking the documentation and tools that the exist real world, I just select flaps 5 and go! Never had a problem with that solution on any runway SWA flys into or out of.

I know about the rwy analysis charts but my doubts remain because for any given rwy usually there are numerous tables with different flap settings for the same wx conditions and weight correct? So in the 737NG for example the crew could theoretically perform a flaps 5 or flaps 15 takeoff given the same atmospheric and rwy conditions and same ATOW. When such a case happens how do you go about favoring one flpa condition over the other.?

Every flap setting has its own MTOW. So the only way you can perform takeoff at the same weight with different flap settings is that your actual weight is low enough to meet both limits.So to choose one setting over the other, the deciding factor (considering all other factors are similar) is...payload! Airlines try to get the more juice out of every flight. The less flaps are used, the more weight can be carried.

Every flap setting has its own MTOW. So the only way you can perform takeoff at the same weight with different flap settings is that your actual weight is low enough to meet both limits.So to choose one setting over the other, the deciding factor (considering all other factors are similar) is...payload! Airlines try to get the more juice out of every flight. The less flaps are used, the more weight can be carried.

Yes, I was considering that the weight in such a scenario would be low enough to meet both limits. This last bit you said was interesting. Somewhere along I got my notions mixed up... I was thinking this whole time that a greater flap setting allowed for heavier takeoffs by increasing lift on the wings. Obviously I need to read up on swept wing acft physics. Care to tell me where to start? :(

It's not the wing sweep that is in play, just basic aeronautics. The flaps increase lift allowing lower Vr thus less runway but increase drag thus less climb, a tradeoff really. At MTOW you want less drag and lots of runway to achieve safe initial climb easily. Shorten the runway and you increase flap to compensate up to a point. In other words, I believe that flaps are used to compensate for runway length rather than weight. Density altitude is also a major factor as well as what I call balanced field length used in small jets that drives the V1 calculation that takes into account accelerate then stop distance required for each runway. All these factors are why the commercial carriers automate this and it is simply fed to the flight crew for each flight.

Yes, I was considering that the weight in such a scenario would be low enough to meet both limits. This last bit you said was interesting. Somewhere along I got my notions mixed up... I was thinking this whole time that a greater flap setting allowed for heavier takeoffs by increasing lift on the wings. Obviously I need to read up on swept wing acft physics. Care to tell me where to start? :(

Hi Victor,My starting point is :less flaps : longer take off distance but better climb rate (important if you need to clear obstacles)more flaps : shorter TO distance but lower climb rate Note : I am only a modest ultralight pilot. See the other comments on other factors (weight/payload etc. ) by more qualified persons on this thread and the other one I pointed to above :( Bruno

Great explanation Dan, thank you.Bruno, what are you flying? I have maybe 150 hours on the Tecnam P-92 Super Echo, which is an absolute delight to fly. I used to know this sort of stuff some years back hehe. Unfortunately I had to give up flying and aviation reading due to my residency... but at least I'm back to flying in virtual skies. Hopefully in a couple of years I'll go back to sportplanes as well. thanks for the input in all of my posts.

In other words, I believe that flaps are used to compensate for runway length rather than weight.

Correct. Pretty much everything that negatively affects lift (read: performance) will require more flaps. Lift is affected by (amongst others) speed, pressure and temperature.Speed is reduced via shorter runway lengths: Less runway means less space to achieve a required speed. When your speed is low you add flaps to compensate for lost lift.Pressure: Lower pressure means less molecules of air going through the wind, which translates into less lift. Which is why you need to fly faster as you go up. If you continue to go up, there will be one point when your stall speed will equal your max speed; that point is called the coffin cornerTemperature: Affects lift in a similar way to pressure. Higher temps will reduce air density which at the same time will reduce pressure. If you takeoff from a runway in summer and once again in winter, you will find that in winter you ate up less runway.All in all, you usually use one set of flaps for standard takeoffs. You select a different one when special conditions are present.

Great explanation Dan, thank you.Bruno, what are you flying? I have maybe 150 hours on the Tecnam P-92 Super Echo, which is an absolute delight to fly. I used to know this sort of stuff some years back hehe. Unfortunately I had to give up flying and aviation reading due to my residency... but at least I'm back to flying in virtual skies. Hopefully in a couple of years I'll go back to sportplanes as well. thanks for the input in all of my posts.

Victor,Mostly a Pioneer 200 (Alpi) but I have flown in Skyrangers, Coyotes and FK9s too. I made a trip in a P-92 with a friend and it's very good too. All the best for your residency ...and I hope you can go back to real flying soon !Bruno

Victor,Mostly a Pioneer 200 (Alpi) but I have flown in Skyrangers, Coyotes and FK9s too. I made a trip in a P-92 with a friend and it's very good too. All the best for your residency ...and I hope you can go back to real flying soon !Bruno

Thanks for the wishes my friend, keep that blue side up!