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Pmdg 737NG - Need Help Decoding These 2 FMC Errors  
User currently offlineGoogles From Kuwait, joined Mar 2013, 14 posts, RR: 0
Posted (1 year 1 week 19 hours ago) and read 5086 times:

Hello.

Got 2 errors on my FMC and not sure how to fix them or how serious they are:

"DESCENT PATH UNACHIEVABLE"
and
"RESET MCP ALTITUDE"

Any help would be appreciated.

6 replies: All unread, jump to last
 
User currently offlineflymia From United States of America, joined Jun 2001, 7174 posts, RR: 9
Reply 1, posted (1 year 1 week 13 hours ago) and read 5065 times:

Both have to do with decent.

Quoting Googles (Thread starter):
"DESCENT PATH UNACHIEVABLE"

This means that the STAR or whatever type of approach you entered into your route from the point you are at now and to the point the route ends the FMC thinks you need to be that the FMC does not think it will be possilbe to reach that altitude on a decent path.
When I get this I just try to slow the plane down as much as possible and take the AP off of VNAV and put it into VS mode to do a much steeper decent, at least at first.

Quoting Googles (Thread starter):
"RESET MCP ALTITUDE"

This just means you need to reset the Altitude number showing on the MCP/Autopilot Panel. For the plane to either climb to the altitude plugged in the FMC the MCP altitude needs to be he same. So if you have set your cruise altitude to FL370 and you want VNAV to take the plane to 370, 37,000 needs to be in the altitude window.
Same idea for decent. If you are 130nm out from landing and you need to get down to 2,000ft then before you hit the Top of Decent your MCP altitude window needs to read 2000 or any altitude under your current one for the FMC/VNAV to start its decent.



"It was just four of us on the flight deck, trying to do our job" (Captain Al Haynes)
User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 2, posted (1 year 6 days 14 hours ago) and read 5017 times:

Quoting flymia (Reply 1):
When I get this I just try to slow the plane down as much as possible and take the AP off of VNAV and put it into VS mode to do a much steeper decent, at least at first.

This is actually counterproductive.

Airplane descents better at high speeds. (per mile flown)



The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
User currently offlinebueb0g From United Kingdom, joined Jul 2010, 642 posts, RR: 0
Reply 3, posted (1 year 5 days 1 hour ago) and read 4957 times:

Quoting Fabo (Reply 2):
Airplane descents better at high speeds. (per mile flown)

Not if you get really slow...



Roger roger, what's our vector, victor?
User currently offlineflymia From United States of America, joined Jun 2001, 7174 posts, RR: 9
Reply 4, posted (1 year 5 days ago) and read 4948 times:

Quoting Fabo (Reply 2):
This is actually counterproductive.

Airplane descents better at high speeds. (per mile flown)

But if I forget to set the MCP altitude to descend and I am only lets say 85nm out of the TOD how would going faster help my decent? The rate of decent I would have to use would eventually speed my plane up anyway but I want to first try to slow her down so I am not passing TOD at a higher altitude. Try to get down under 300kias where the spoilers might actually make a difference too.



"It was just four of us on the flight deck, trying to do our job" (Captain Al Haynes)
User currently offlineSpeedbird128 From Pitcairn Islands, joined Oct 2003, 1648 posts, RR: 2
Reply 5, posted (9 months 1 week 1 day 6 hours ago) and read 2800 times:

Quoting flymia (Reply 4):
But if I forget to set the MCP altitude to descend and I am only lets say 85nm out of the TOD how would going faster help my decent? The rate of decent I would have to use would eventually speed my plane up anyway but I want to first try to slow her down so I am not passing TOD at a higher altitude. Try to get down under 300kias where the spoilers might actually make a difference too.

From cruise level 85nm out, you'll need to fly a hold or two - descending the entire time with spoilers out is annoying for passengers   



A306, A313, A319, A320, A321, A332, A343, A345, A346 A388, AC90, B06, B722, B732, B733, B735, B738, B744, B762, B772, B7
User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 6, posted (8 months 1 week 6 days 12 hours ago) and read 2174 times:

Quoting flymia (Reply 4):
But if I forget to set the MCP altitude to descend and I am only lets say 85nm out of the TOD how would going faster help my decent? The rate of decent I would have to use would eventually speed my plane up anyway but I want to first try to slow her down so I am not passing TOD at a higher altitude. Try to get down under 300kias where the spoilers might actually make a difference too.

Late reply, but... well.

Thing is the efficiency of glide is the best at low speeds. About 200-220ish clean wing typically (green dot in Airbii).

That means then, that for a mile flown, you will have least feet descended. If you go 300kts, you will descend faster. For example, if on 210 knots, you would descend 700fpm, on 300 knots, you would descend 1500fpm. That means 200ft/mile compared to 300ft/mile.

Spoilers are more effective in higher speeds as well. More speed=more drag=more descend.



The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
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