The climb performance - is it normal?

[Georg L]

Hi,

So I made a flight with the 733, upper winds were pretty calm, about 20 kts tailwind. The ZFW was 46 t, 7 tons of fuel on board. I had a cruise FL calculated to be FL360 and so said the FMC as optimal. Cost index 20 and the aircraft climbed pretty well until about FL230, then the climb performance was reduced a lot and it kept going up at around 500-1000 fpm maximum. Sometimes it completely halted the climb to gain some airspeed again. Very often the IAS was about 20-30 kts less than requested by FMC. Above FL300 the V/S went up to 1500-2000 fpm occasionally, then reduced agin to 0 or 500-1000.

I'm asking this because I'm a real life ATC and we have several 735 and 733's flying around. I always see them climb about 2000-3000 fpm up to FL200 and then it continues to climb 1000-2000 fpm all the way up to FL360. They never reduce the rate to 0 to gain some IAS but perhaps rarely keep going at 1000 max when they are heavy. It just seems their climb performance in real is much better on higher levels.

Any ideas?

Otherwise just awesome aircraft and certainly above the rest in so many ways.

Thanks,

George

[Morten]

32 minutes ago, speedbird1229 said:

I'm asking this because I'm a real life ATC and we have several 735 and 733's flying around. I always see them climb about 2000-3000 fpm up to FL200 and then it continues to climb 1000-2000 fpm all the way up to FL360. They never reduce the rate to 0 to gain some IAS but perhaps rarely keep going at 1000 max when they are heavy. It just seems their climb performance in real is much better on higher levels.

Hi George,

It might be our max climb N1 limit is set a bit low, we'll look into that!  Doesnt sound right.  As for your real observations, they are not quite accurate (statistically).  According to official BADA data (Data used for ATC Management) a B733 at nominal ("average") weight climbs at less than 1000 fpm above FL330 (statistically).  FPM offcourse also has to do with the engine rating, cost index etc. and we have the lowest rated _B1 model. 

Thanks for the report.

M

[Georg L]

Thanks! It could be that most of the 737-300's I see climbing are doing short haul hops but then again the need to level off temporarily and gain more airspeed seems a bit too out of the envelope... It can certainly be the N1 limit too.

What do you think about the VNAV not really holding the exact specified IAS on climb? Is it supposed to behave like this?

[g650flyer]

Another thing that can also hinder your climb is climbing with indicated airspeed instead of switching over to mach. The higher you fly, the faster TAS you need for a given IAS. Once you climb in mach, the TAS decreases. Most changer overs occur some where between 27 to 29 thousand when recommended IAS climb speed intersects recommended mach climb speed

[Georg L]

2 minutes ago, g650flyer said:

Another thing that can also hinder your climb is climbing with indicated airspeed instead of switching over to mach. The higher you fly, the faster TAS you need for a given IAS. Once you climb in mach, the TAS decreases. Most changer overs occur some where between 27 to 29 thousand when recommended IAS climb speed intersects recommended mach climb speed

Yep. This shouldn't be the issue in my case since I always used the VNAV climb which should automatically transition I believe. That's why the V/S increased a bit more above FL300 I guess.

I don't mean the climb rate was that bad but it was also very weird that it needed the level offs. Perhaps it's somehow related to the autopilot behaviour and it's ability to hold the IAS or Mach on climb.

[g650flyer]

1 hour ago, speedbird1229 said:

Yep. This shouldn't be the issue in my case since I always used the VNAV climb which should automatically transition I believe. That's why the V/S increased a bit more above FL300 I guess.

I don't mean the climb rate was that bad but it was also very weird that it needed the level offs. Perhaps it's somehow related to the autopilot behaviour and it's ability to hold the IAS or Mach on climb.

I only mentioned it because you mentioned IAS and IAS speed loss. I can say it also depends on your altitude you are climbing to. I've flown light to heavy jets and this is the typical behavior i've seen in regard to climb performance. In regards to optimum, I have always been in operations where you climb to 2000ft above optimum. If step climbing, I hang out till 2000ft below and repeat the process. If doing a constant cruise operation, again, I plan 2000ft above optimum. In these cases, I fly the recommended climb speeds and don't stray too far from it even if in a hurry. I've watched guys climb at too fast of a climb speed and couldn't make it until they reduced speed. On average when near optimum and above it, I see  climb rates of 500 to 800 feet per minute. When climbing up to the aircraft's service ceiling, its about 300 feet per minute. When I see these type climb rates, I know i'm in the optimum altitude range and approaching the aircraft's performance limits. As you know, you don't want to be below the 500 feet per minute mark for ATC reasons. Here's a good real world operating tip. When climbing in mach in close to optimum, the plane tends to get pitchy when in VNAV or FLCH. When the plane starts to get loose in pitch, we select vert speed to settle it down. When in vert speed, you work the vert speed to maintain your climb speed. You can get in trouble really quick if you let the plane pitch too much. If you get behind the thrust curve, it can take a long time to get back on speed and climbing again. Plus the pitching gets aggressive and leads to overspeed or stalling. If the 737 is climbing correctly, these are good rules of thumbs to fly by.