Aircraft Performance

[United744]

I'll let the screenshot speak for itself.

Does the aircraft really have this much performance at this altitude? It was steady, and not the result of a wind shift. Whilst it is slightly faster than climb speed, the sped decay was very slow (approx. 1 kts/5 sec). Climb speed was set to 300 kts/M 0.83.

[Will]

A screenshot can't really speak for itself in this case, because a screenshot is a frozen moment in time. You would need to see trends evolve over time before you could comment on performance.

In this pic, the commanded climb speed is M0.83, while the actual speed is M0.841. That means you're flying faster than you're telling the aircraft you want to fly. Meanwhile, the vertical mode selected is VNAV SPD, which means that the AFDS will use the aircraft pitch to maintain the commanded speed.

Put it all together now: you're commanding a speed slower than you are currently flying, and the aircraft is using pitch to achieve the command speed. What happens? Nose up, until the command speed is achieved.

This kind of climb is called a "zoom," when you trade airspeed for altitude (or when you trade kinetic energy for potential energy). That can result in an impressive climb rate while the speed bleeds off, but that impressive climb rate is momentary, not sustained. Once the actual speed reaches the command speed, you'll see a much lower climb rate and a lower performance.

So in sum: brief zoom climbs can give you impressive rates, but that's just until the airspeed-for-altitude trade is complete. when the aircraft is flying at the command speed, the climb will be shallower.

[United744]

Understood, but the aircraft was hardly decelerating. The Mach number didn't reduce by level off at FL370.

At FL210 she was climbing at 3000 ft/min.

At 7500 ft she was climbing at 4000 ft/min.

 :shock:

I wanted to climb to get above the terrain (peaks at 23000 ft), but I wasn't expecting this.

Time to climb was 16 minutes to FL370 (and that included a 2 minute level segment at 9000 ft, so really it was 14 minutes to FL370). Not bad for 610,000 lbs! Just seems a bit fast. If she'll do it though, I'm not complaining!  I'll make more use of de-rated climb power. Note this is with the RR engines.

EDIT: Hmm that is average of 2600 ft/min. Not beyond the realm of possibility.

[Hardy Heinlin]

In addition to Will's hint, also note that while your commanded Mach number remains constant during the climb, your commanded KIAS will be gradually decreasing. The PFD airspeed tape is a KIAS tape, not a Mach tape. There is always a KIAS drift during a fixed Mach climb/descent.

[United744]

Interesting. So even though it should be a Mach 0.83 climb, it had settled for a M 0.84 climb? Is there any tolerance to the climb speed/Mach number that it targets so it isn't chasing every tiny change?

[Hardy Heinlin]

If the command speed is a Mach number, the elevators maintain the Mach number.

If the command speed is an IAS, the elevators maintain the IAS.