"Unable Cruise Alt" Collins FMS 6000

[TKS850]

Attention all FMS Performance Guru's !!

Has any Collins FMS 6000 operators found problems with the predicted climb performance/cruise alt of their FMS ?

In my case our HS125-800 FMS generates an "Unable Cruise Alt" msg on a regular basis. The main problem is that the Flight manual Cruise Alt V's Weight charts, for the same weight/flight level and ISA deviation don't seem to match with the FMS msg warning. What am I missing Here ??

When the FMS is calculating the Performane Cruise Ceiling, is it only concerned with Flight Manual Ceiling limitations or is it also looking at buffet boundary ?
Should I observe the FMS warning and select lower levels or select levels with respect to the Flight manual charts and ignore the FMS warning ?

Any input much appreciated.

[Gen. Jack D Ripper]

Suggest you ask the Engineers to look at the level of 'G' protection set in the box. It may be set at (for example) 1.1G whereas your aircraft may be perfectly capable of 1.3 or 1.6G.

Let me know the outcome.

Cheers

[3bars]

On our FMS, if you select a low Econ Index, you get this message advising a lower more efficient cruise level. Not sure if the Collins 6000 is the same.

[fanspeed]

Check the Defaults page and make sure the cruise settings match those you are using. Good luck!

[downwindabeam]

I've seen our CRJ Collins FMS produce that message for a variety of reasons.
(Bare in mind FMS on CRJ-200 is older than FMS-6000 I believe)

1) Unable what is set as cruise in PERF INIT because the VNAV planned will not let you reach CRZ ALT before reaching another altitude constraint you put in the LEGS page whether it be on the way up or on the way down. Sometime putting the ILS on a short flight which brings with it the fixes with their altitude constraints will produce an "UNABLE CRZ ALTITUDE" msg.

2) YES, It seems FMS tables are different in many aspects from the books. Seems like it interpolates more than the tables did. Flex temp would be a good example of that.

3) I've seen it come before with no rhyme or reason, a couple times it seemed like temp affected it tho not sure what was so unusual that day.

[eckhard]

You might want to check FMS settings for:

1. Default setting for cruise CofG;
2. Default setting for max thrust in the cruise. It could be 'Cruise', 'Climb', or 'Max Cont';
3. Buffet boundary 'g' limits (as suggested previously); and
4. Actual engine model and thrust rating.

I don't know much about the FMS 6000 but these parameters are relevant on other FMSs.

Good luck!

[spoilerons]

Quoted from the FMS-6000 for the Challenger 604 Pilot's Guide:

UNABLE CRZ ALT: FMS performance predictions indicate the airplane cannot reach the cruise altitude because of climb or cruise ceiling limitations, or the climb and descent profiles overlap. Correct the vertical profile to clear the message.