Jump to content

Sign in to follow this  
Guest mccaba

747-400 looses airspeed

Recommended Posts

Guest mccaba

Yesterday I was flying a transatlantic route KJFK - EGLL. I had set everything up using the PMDG load editor and correct fuel etc At first I had some probs climbing to my CRZ of FL370. I reached about 35,000 and then it just refused to climb any further. Then I started to gradually lose airspeed. The autothrottle seemed to be trying to compensate, but even with 106% N1, and EPR going off the scale, the plane gradually lost speed. I disengaged the autothrottle, but still could not regain airspeed. BTW, I use AS6 which was reading a tailwind of over 100kts at the time. Any ideas ?Andrew

Share this post


Link to post
Guest Babubhai

What IAS were you climbing at? And what was your aircrafts weight? Looks like your aircraft was too heavy to climb to that altitude, thats why you have step climb techniques. Do a search on step climbs in this forum...

Share this post


Link to post
Guest mccaba

Cant recall the detail, but airspeed was as dictated by the FMC. My airspeed was falling steadily and was down at about 240 kts IAS despite being at 106% N1. The Alt was within the range shown as Opt or Max in the FMC and a step climb wasn't needed according the the FMC. Andrew

Share this post


Link to post

Do you have wind smoothing enabled? Was the wind changing at the time?I find that even with wind smoothing enabled, when changes occur and they result in the airspeed decreasing, it can be a major problem trying to maintain speed, espcially when climbing. I've had one or two flights where I've had to descend during the climb phase to stop the airspeed plunging into the stall zone. Tends to be is the speed increases from 40kts to say 120kts. As I have FSUIPC set to smooth by 1 degree/knot per second, it can take 80 seconds for the wind to settle, all the while I lose 1 knot of airspeed for every knot/degree change of wind (due to the unrealistic way FS models wind/airspeed interaction).You could try increasing the time for each knot/degree change in wind, but this means when landing, you can sometimes be on the ground and the wind is still trying to achieve what the surface wind should be. But it will give the aircraft a chance to try and keep up with the changes.Only consolation is that the wind rarely changes by such a large amount, which is what can cause these problems.


Phil Brown

Share this post


Link to post

Strong tailwinds are indeed tricky assuming you're not too heavy for your current alt. Happened to me once or twice - did not realize at the time I should have set thrust limit to CLB or CON for cruise phase (read that someplace - it's done in real world).Doing this has since prevented me gradually losing indicated airspeed in strong tailwinds. I think you have to prevent it early, or your only hope is descent to regain speed.regards,Mark


Regards,

Mark

Share this post


Link to post

Not sure if this is off topic, but I find that the autothrottle does not work the way I expect, i.e. maintain the FMC speeds. I have to use the manual throttle to maintain the speed bug indication, although it does appear to work in autoland situations. Am I doing something wrong?Ron


Ron Service

.

Share this post


Link to post
Guest mccaba

Well I made a short flight today and tried to get this problem to occur again, but to no avail. I'm pretty sure the weights were OK the day I had the prob and my 747 climbed to its crz alt today with no problem. I'll try that tip about setting thrust to CLB or CON when I have a tailwind next time. Andrew

Share this post


Link to post
Guest TSE

HiI saw this happening during one of my recent Atlantic crossings.I think it may be related to ASV and the OAT at CRZ LVL, displayed OAT was much higher than normal during the occurence (only about -5 degrees) and the engines were fighting to maintain the speed set in the FMC.I was about 2000 FT above the optimal CRZ LVL due to weather at the time this happened.Once ASV switched to YYT as the active weather station, the problem disappeared and I was able to maintain M 0.86 with about 93 % N1.Best regardsJoerg

Share this post


Link to post
Guest mccaba

>Strong tailwinds are indeed tricky assuming you're not too>heavy for your current alt. Happened to me once or twice - did>not realize at the time I should have set thrust limit to CLB>or CON for cruise phase (read that someplace - it's done in>real world).>>Doing this has since prevented me gradually losing indicated>airspeed in strong tailwinds. I think you have to prevent it>early, or your only hope is descent to regain speed.>>regards,>MarkWell, it just happened again. At my CRZ alt with a tail wind of 63kts and started to lose airspeed despite 100% N1. So tried changing Thrust Limit to CON, and hey presto, started to gain speed even with a lower N1 % and resumed normal crz speed. Thanks for the tip Mark.Drew

Share this post


Link to post

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this  
×
×
  • Create New...