Regardless of what altitude the AFM or FMS predicts the airplane is capable of achieving at a given WAT, they do not account for unanticipated atmospheric conditions such as mountain wave, CAT or warmer than predicted (the temps entered in the FMS or used on the perf chart.) temperatures. If any of these conditions are encountered, then it will be important that the crew recognize the degraded performance and act appropriately. There comes a point in this scenario of declining speed (VS mode) or ROC (IAS,MACH or FLC mode) when defeat must be admitted! Time to level off or if getting really slow, descend. Will FLC descend to remain on speed? Airplane performance charts are done as part of certification and are intended to provide a means of performance planning. They guarantee nothing if the conditions are different than stated. You make your plan based upon the available information and then see how it goes. The ASI and VSI will let you know how it is going. The OAT, TAT, RAT or SAT will help you recognize whether temperature is the culprit. AOA can be used to crosscheck the speed trend. All of these things are available to jet pilots (depends which jet) for good reason. It is up to pilots to observe airplane performance closely and be careful what we believe. Always view AFM or FMS predicted performance as possibilities not yet proven. Show me! It is also better to let ATC in on your predicament earlier rather than later.
The above words might just as well have been written before the occurances being discussed on this thread. The lessons were learned because they came up as a natural consequence of flying moderate (some might say weak performers!) performance jets. I have little interest in magazines or crosswords while in flight. I do however enjoy watching all the little dials and indicators and such. I used to think all pilots did. Another useful lesson learned. (cool pilots read magazines) I guess my natural curiosity and interest pleases me at least. When it doesn't, I'll work for Wyvern or FAA. 9 to five baby! But not just yet.
Best to all,
Westhawk