Jump to content

Sign in to follow this  
OMillerJr

Descent problem

Recommended Posts

I had set the 1st waypoint at 10,000 ft and the 2nd at 5,800 ft. The 2nd waypoint was 29 miles from the 1st. The aricraft descended to 10,000 ft properly and as I passed the 1st wapoint it started to descend to the second normally at somewhere around 1,400 to 1,800 ft/min. After a short distance the descent rate decreased to approx. 100 to 200 ft/min. By the time I was nearly on top of the 2nd waypoint the plane had only dropped to about 8,500 ft. About 1/2 way between the 1st and 2nd waypoints I had the speed down to 155 and 25 degrees of flaps. Just before or as the plane started to make the turn at the 2nd waypoint I got the FMC message "UNABLE NEXT ALTITUDE" The 3rd waypoint was set at 3,800 ft and was 17 miles from the second. The aircraft started decending rapidly (don't remember the descent rate) but it did make the 3,800 ft or thereabouts and I was able to engage the Localizer and Glide Slope and land properly. I've had this happen before. The question is, why wouldn't the plane descend the necessary 4,200 ft between the 1st and 2nd waypoints? All speeds and altituds were entered manually (show up as large font.)Oh yeah, forgot to mention, this is on the 747-400.

Share this post


Link to post
Guest Hawkeyeted

When you set up the FMC for the second restriction, what speed did you use (i.e. what speed was next to the restricted waypoint altitude?). If it was anything other than 155 kts, (that's what you said you were flying) then that may be your problem. The FMC may have calculated a 240 kts descent to the 2nd restriction, but you were manually flying at 155 kts.

Share this post


Link to post

You are correct, it was showing 240 but I've never had to set the speed before and it usually works properly. I'll try setting the speed next time and see what happenes. Thanks for the response.Owen

Share this post


Link to post
Guest Hawkeyeted

155 kts is a LOOONG way from 240. Perhaps in the past you were closer to the planned speed ? (i.e. 220 vice 240?)

Share this post


Link to post

Agreed, 155 is a long way from 240. I just had another occurance and I did have the speed set at 155. The aircraft just eased down at approxamatly 100 ft/min. I could see it wasn't going to make the next planned altitude. For what ever reason I pushed (clicked) the speed setting knob and the 155 came up in the speed window, the plane took a nose dive and caught up and passed the next waypoint at planned altitude. I did not manually change the speed after I pushed the set knob. I'm really confused. Don't know what the connection is, I just set the speed to manual. Part of the dive was, I assume, because VNAV SPD was trying to make speed catch up. It had dropped 8 - 10 knots below 155 since the throttle on the FMC was HOLD. When the speed regained 155 knots the descent went back to somewhere around 1,500 - 1,800 ft/min and seemed to continue normally to the next altitude. Sorry, I know I'm not explaining this very well.Owen

Share this post


Link to post
Guest Andrea

If I remember correctly the problem of not respecting many alt/speed fixes (even though they are realistic) during descent is a known bug of the PMDG 737NG.Andrea

Share this post


Link to post

I feel like it may be a bug, but I don't know. The reason I've been setting the speed/altitude restrictions to force a more shallow descent is because with the automaticly calculated FMC speed/altitude settings the descent seems to be much to steep and even though it may be realistic I don't want to have to constantly use the speed brakes to control the speed (slow down.) I haven't flown that much and never in a 747, but is it normal to have to use speed brakes so much to control the speed?Owen

Share this post


Link to post
Guest Andrea

But with the B737 the use of speed brakes during descent is quite usual in reality.Andrea

Share this post


Link to post

>If I remember correctly the problem of not respecting many>alt/speed fixes (even though they are realistic) during>descent is a known bug of the PMDG 737NG.>>Andrea"Oh yeah, forgot to mention, this is on the 747-400.[/b}"[h4]Randy J. Smith[/h4]AMD 64 4000+|ASUS K8V DELUXE|SAPPHIRE ATI X800XT PE|MUNCHKIN 3200|80 gig SATA|DELL 1905FP 19" LCD|TRACKir PRO|PFC JEPPESEN MOONEY YOKE|CH PRO PEDALS|


Randy J Smith

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  
  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
  • Donation Goals

    AVSIM's 2020 Fundraising Goal

    Donate to our annual general fundraising goal. This donation keeps our doors open and providing you service 24 x 7 x 365. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. We reset this goal every new year for the following year's goal.


    2%
    $565.00 of $25,000.00 Donate Now
×
×
  • Create New...