Hi guys,I'm currently on enroute with SWA131 from Orlando to Phoenix. Since the take off I had a very hard time with about 60 kts headwind and thus using more fuel than I thought of. That's why I did configure an step climb from FL380 up to FL400. The FMC showed me about 4% saving from this but instead of making the climb soon after I entered it into the system but rather 500NM later. I have now done that climb and it was the right decision: now I've an fair tailwind of about 10kts and saving some fuel.So my question now is: How does the plane know when to make that step climb?

nevermind

Weight + Lift + Drag + Speed + power = how high you can climb.The obvious thing being, as you use fuel you get lighter (cos that heavy fuel isn't in your tank anymore).The FMC knows how much your aircraft is (you told it in the Init Ref page).The FMC knows how much fuel you have (fuel sensors)The FMC knows how to do maths (Total weight before minus Weight of used fuel - total weight now)And even more complex maths (How much air pressure is at FL380 vs FL400, and how much lift can the wing make at this speed at that airpressure)It will also predict how much power the engines can deliver at altitude. The higher you climb the less air there is, thus less air particles for the wings to produce lift, and less air particles going through the engine to make Thrust.Now what happens if you go too high too soon?

http://www.youtube.com/watch?v=FMrQp9c6_MM

Regards, Trent Hopkinson

I believe the FMC takes into weight and then finds when your step will coincide with the climb to the MAX CRZ at that weight. It might be looking at OPT ALT, but it seems like the FMC always wants you to climb to the STEP before it's close to the OPT. Maybe it gives a climb above the OPT when being below it more costly. Dunno.If you are experiencing turbulence, you might want to hold off on your step climb until the air has smoothed out or the OPT has climbed a bit. Otherwise you get a bit to close to the buffet speed for most pilots comfort. It hasn't been a very useful tool for me. The FCTM has a short section on this.

I believe the FMC takes into weight and then finds when your step will coincide with the climb to the MAX CRZ at that weight. It might be looking at OPT ALT, but it seems like the FMC always wants you to climb to the STEP before it's close to the OPT. Maybe it gives a climb above the OPT when being below it more costly. Dunno.If you are experiencing turbulence, you might want to hold off on your step climb until the air has smoothed out or the OPT has climbed a bit. Otherwise you get a bit to close to the buffet speed for most pilots comfort. It hasn't been a very useful tool for me.The FCTM has a short section on this.

I believe (have observed) That the step climb is calculated by the FMC to be allowed to climb when Optimum is still 1000ft below the next altitude. Ie if you are at FL370 and wish to step climb to FL390, the FMC will allow a climb when Optimum is FL380.As the above video shows, It may be an idea to hold off on that climb if there is turbulence or icing till Optimum is actually at FL390 to give more of a speed range between high and low speed buffet (yellow line on speed tape in the NGX).Trent Hopkinson.

Weight + Lift + Drag + Speed + power = how high you can climb.The obvious thing being, as you use fuel you get lighter (cos that heavy fuel isn't in your tank anymore).The FMC knows how much your aircraft is (you told it in the Init Ref page).The FMC knows how much fuel you have (fuel sensors)The FMC knows how to do maths (Total weight before minus Weight of used fuel - total weight now)And even more complex maths (How much air pressure is at FL380 vs FL400, and how much lift can the wing make at this speed at that airpressure)It will also predict how much power the engines can deliver at altitude. The higher you climb the less air there is, thus less air particles for the wings to produce lift, and less air particles going through the engine to make Thrust.Now what happens if you go too high too soon?

http://www.youtube.com/watch?v=FMrQp9c6_MM

Regards, Trent Hopkinson

Love the crew reaction in the video. "We can't accelerate.""lol. I gotta take a whiz."

Love the crew reaction in the video."We can't accelerate.""lol. I gotta take a whiz."

Yeh their CRM was pretty ... non existent.Hence the crash I spose.

Ok guys, when the step climb is made is based upon the actual weight. Thanks for that. I guess I just had luck that the weather was a lot nicer on FL400 than on 380.

Ok guys, when the step climb is made is based upon the actual weight. Thanks for that. I guess I just had luck that the weather was a lot nicer on FL400 than on 380.

Depends on where you were. Typically wind direction doesn't change 180 degrees in 2000ft. Sounds more like in the time that passed waiting to step, you passed a weather system.The only time I step climb is when I have a good tailwind and forecast winds are stronger at higher altitudes (ie when the jetstream is working with me)