PW4062 Engine Performance/Thrust at High ISA Delta

[United744]

Hi,

Does anyone fly very often with the PW4062s?

Despite these engines being the most powerful available, they don't seem to like low speed, and aircraft acceleration is lower vs. RR or GE options as a consequence.

I'm curious on several levels.

Lack of power at low speed I can understand (ram-air effect and inlet design being the most obvious), but what I can't quite understand is why the poorer performance at altitude?

The last few nights I've been exploring the new weather features, and I've been flying in more extreme environments than I normally would (very warm/cold) which has made the differences more apparent.

Flying in South America last night (western side between Ecuador and Bolivia), temperatures were what I would call very warm, at ISA+26 at FL350 (real-world weather; not manually set).

The aircraft was nearing max landing weight at 620,000 lbs (281 tons) but wasn't what I'd call particularly heavily loaded.

The aircraft could not climb to FL350 at all, despite the FMC being loaded with weather data, and VNAV CRZ showing an optimium cruise level of 350.

Between allowing the aircraft to stabilize with the PWs, I selected the RR and GE engines. Neither of these had any issues climbing to FL350.

As soon as the PWs were selected, airspeed reduced, and it was unable to climb.

The aircraft was not slow - it was climbing at M 0.86, which I find helps the PWs. They do not like getting slow at all, at any time.

Why is there such a difference between reported engine max thrust capability, and actual performance?

When ISA delta is this high, does anyone know what the cruise correction should be? It appears the FMC does not consider forecast OR actual temperature in its optimum prediction, and so there must be table(s) the pilots use to know what the correction should be to cruise altitude in this situation?

[Hardy Heinlin]

It appears the FMC does not consider forecast OR actual temperature in its optimum prediction ...

It does. It's a mixing algorithm using current data and forecast data.

[andmiz]

FMC optimum altitude should not take wind or temperature into account.  It%u2019s only determined by the gross weight vs speed mode (when in a speed mode other than ECON), or gross weight vs cost index (when in ECON).

Maximum altitude would be our more useful calculation in these scenarios, which does take into account the temperature.

[Hardy Heinlin]

Right, I agree. I was just thinking of the OAT for the command Mach in ECON mode, as the TAS/Mach ratio is OAT dependent.

[United744]

It appears the FMC does not consider forecast OR actual temperature in its optimum prediction ...

It does. It's a mixing algorithm using current data and forecast data.

I thought so.

In that case...why was it struggling to even maintain speed, never mind climb?