Simviation Forums

Flight Details
-default Boeing 747-400
-IFR KSEA to RJAA
-FL34,000
-weather: real world with 15 minute updates (currently fair)

Problems
1) While using the A/P and A/T, the engine power, throttles, and IAS are all over the place.

2) Bringing up the Throttle Quandrant, I notice the A/P almost continuously adjusting power for several minutes, then leaving the throttles at an "incorrect" setting (too fast or too slow) for extended periods of time only to correct the problem later after another period of much adjustment.

3) IAS is varying over about a 70 knot range.  What is very concerning is when the aircraft previously surged past its overspeed limit despite having the Speed Hold Selector set below the maximum speed for the aircraft.  I lowered the Speed Hold Selector to 300 KIAS and still get surges to up over 330KIAS.

Hypotheses
1) There are changing strong headwinds and tailwinds that the aircraft is passing through, and the A/T is having difficulty adjusting quickly to the prevailing wind conditions.

2) The FS has developed a glitch and rendered the A/P only partially operative.

Questions
1) Has anyone else experienced significant increases and decreases in engine power while using the A/T on the default airliners?

2) Any ideas as to what is the actual cause for these mysterious fluctuations in engine power?

Thanks,
~Darrin

I think you're right. I don't fly the big stuff enough to have another opinion..  

Even flying a single-engine prop at 8000msl, you can get some pretty significant shifts, and the airspeed indicator needle jumps rather sharply. Since the A/T gets its input from the airspeed indicator, I'd imagine the "throttle servo" has trouble keeping up.

The only thing you didn't mention, is if you have "download winds aloft" checked. I don't know if that would matter, but binary logic calls for asking (if yes, try no... if no, try yes), and then see what happens.

Also... if there's no problem flying in "clear all weather", that would rule out an A/P A/T glitch.

Winds aloft will definitely make a difference a head or tail wind can cause the a/p some problems keeping the correct speed. A strong head wind will make the airspeed indicator show a faster speed than what you are actually flying. While a tail wind will push you and thus cause the Auto throttle to turn down the power. If you use FSNAV it will show you the winds aloft, during this time of year a wind blowing from the west is not unusual and they can get rather high at times. I have seen 65 and 75 knot winds out of the west. And believe me that will make the auto pilot surge. About all you can do is to either climb above it or go below it.  8-)

Winds aloft will definitely make a difference a head or tail wind can cause the a/p some problems keeping the correct speed. A strong head wind will make the airspeed indicator show a faster speed than what you are actually flying. While a tail wind will push you and thus cause the Auto throttle to turn down the power. If you use FSNAV it will show you the winds aloft, during this time of year a wind blowing from the west is not unusual and they can get rather high at times. I have seen 65 and 75 knot winds out of the west. And believe me that will make the auto pilot surge. About all you can do is to either climb above it or go below it.  

You got it a little backward there..  All the A/P cares about is your airspeed, so whether you're being pushed or pulled (relative to the ground) is irrelevant...  It's a wind shift, and ensuing change in airspeed that gives the A/P fits. A shift to a tailwind might increase your ground speed.. BUT it decreases your airspeed so the A/P increases the thrust after a shift to a tailwind.

A few suggestions, Darryn:-

I lowered the Speed Hold Selector to 300 KIAS and still get surges to up over 330KIAS.

1. Over 300KIAS is FAR too fast for the 737. It remains one of my FS favourites but she's an elderly lady now! The Learning Centre recommends a cruising speed of Mach 0.74 which is about 275KIAS at 35,000 feet. 'Over 300KIAS' would be around Mach 0.85 which no 737 ever built would be comfortable with! Strongly recommend using Mach speeds above 25,000 or so, anyway. Why ever not?

2. Quick way of checking wind speed/direction is to press 'Shift+Z,' which brings the figures (plus other useful info.) up in red at the top of the screen. Keep pressing 'Shift+Z' to clear.

3. Make sure you have 'pitot heat' switched on before you take off - leave alone at 35,000 feet! Otherwise the pitot head is ('virtually') certain to freeze up and start showing some really cockeyed speed variations. Maybe check the outside air temperature too, you might need engine heat as well.

Xyn_air,

I think a good part of the issue is that you are not shifting to utilizing Mach at the altitudes you are running on trans-Pacific flight.  Remember you are looking at IAS ..........INDICATED airspeed.  Up there in the tippy top of the Earth's atmosphere there are few air molecules hanging out so the pitot tube is not seeing the same number of molecules pounding into the opening... hence seeing lower pressure differential between it and the the static port...... and it THINKS that the plane is going a LOT slower than it actually is.  Rermember to shift the MCP to Mach mode.

I don't fly that plane, but look up the maximum Mach speed it is designed for... and remember that is the MAXIMUM.  You likely will not cruise at that speed.

When you start to approach the aircraft's maximum speed, you start to get high speed buffet.  That also might be a part of what you are experiencing.  Research the phrase "Coffin Corner".   8-)

best,

....................john

PS:  The trans-Pacific route is KNOWN in the RW for the impact of the Jet Stream on ground speed.  USA to Narita is noticably slower due to the headwind than Narita to USA by a significant factor.   My usual RW route RJAA - KJFK is about 1 1/2 hours shorter time-wise on the way back to the USA.

Xyn_air,

I think a good part of the issue is that you are not shifting to utilizing Mach at the altitudes you are running on trans-Pacific flight.  Remember you are looking at IAS ..........INDICATED airspeed.  Up there in the tippy top of the Earth's atmosphere there are few air molecules hanging out so the pitot tube is not seeing the same number of molecules pounding into the opening... hence seeing lower pressure differential between it and the the static port...... and it THINKS that the plane is going a LOT slower than it actually is.  Rermember to shift the MCP to Mach mode.

I don't fly that plane, but look up the maximum Mach speed it is designed for... and remember that is the MAXIMUM.  You likely will not cruise at that speed.

When you start to approach the aircraft's maximum speed, you start to get high speed buffet.  That also might be a part of what you are experiencing.  Research the phrase "Coffin Corner".   8-)

best,

....................john

PS:  The trans-Pacific route is KNOWN in the RW for the impact of the Jet Stream on ground speed.  USA to Narita is noticably slower due to the headwind than Narita to USA by a significant factor.   My usual RW route RJAA - KJFK is about 1 1/2 hours shorter time-wise on the way back to the USA.

Everyone, thanks for the thoughts and advice.  It has given a lot of things to mull over.

John, as always, thanks for getting me thinking and learning.  I seem to remember hearing somewhere, now that you mention it, about Mach value being used over KIAS at high altitude and for faster speeds for all the reasons you so aptly stated.  I also looked up the "coffin corner".  That turned out to be a great subject as it lead to several other informative subjects.  Thanks for mentioning it!

Well, even as FS makes it, sometimes moving those airlines around is still a challenge.  It really makes me appreciate how much more responsive and forgiving lighter aircraft can be at times.  Meh, either way, I am glad I get to experience both in FS.  And with practice and great advice, I will probably get even better as time goes on.

Appreciatively yours,
~Darrin

A few suggestions, Darryn:-

I lowered the Speed Hold Selector to 300 KIAS and still get surges to up over 330KIAS.

1. Over 300KIAS is FAR too fast for the 737. It remains one of my FS favourites but she's an elderly lady now! The Learning Centre recommends a cruising speed of Mach 0.74 which is about 275KIAS at 35,000 feet. 'Over 300KIAS' would be around Mach 0.85 which no 737 ever built would be comfortable with! Strongly recommend using Mach speeds above 25,000 or so, anyway. Why ever not?

Well, it is a good thing I was flying a 747-400, then.  ;)

EDIT:
I just noticed that while in the O/P I stated I was flying a 747, the thread title says 737.  Yikes!  How embarrassing for me!  Now, how do I change that?!?  :o :-/

Fixed.  Still pretty embarrassing, though.  Heh, sorry about that, Nav.

2. Quick way of checking wind speed/direction is to press 'Shift+Z,' which brings the figures (plus other useful info.) up in red at the top of the screen. Keep pressing 'Shift+Z' to clear.

Thank you very much for this little bit of information!  I was wondering in mid-flight if there wasn't some way to check on local wind conditions.  That was bugging the heck out of me for a bit now.  I mean, normally I fly light aircraft near to airports, so I can always tune in to the ATIS for information.  But, I wasn't really sure how to get that info when out over the ocean at FL34,000!  Thank you!  

3. Make sure you have 'pitot heat' switched on before you take off - leave alone at 35,000 feet! Otherwise the pitot head is ('virtually') certain to freeze up and start showing some really cockeyed speed variations. Maybe check the outside air temperature too, you might need engine heat as well.

You know, I was looking all over the default 747 cockpit for the pitot heat switch and I could not find it.  The only thing I could find was the anti-ice switch.  Now, I thought that the anti-ice switch was for all-over anti-icing, not just the pitot.  But, is this what one would use anyway when flying the default 747?

Thanks for the helpful advice, Nav.  Sorry, but I couldn't help but tease you just a little about the first mis-read about the '737'.  Just so you know, I wasn't miffed in the least, and got a good chuckle out of, as I hope you do, too.  But, back on point, thank you very much.  (Well, before I teased you, I probably should have checked that typo in my subject line.  :-[)  Today has been one of those days where I am learning a lot, and I am very grateful for it.

All the best,
~Darrin

2. Quick way of checking wind speed/direction is to press 'Shift+Z,' which brings the figures (plus other useful info.) up in red at the top of the screen. Keep pressing 'Shift+Z' to clear.

Xyn_air,

Another possibility:

Not sure which model you are flying and the realism factor there, but if you look on the ND (Navigation Display) ....usually the inboard (right) side of the Captain's main MFDs, on most Boeing displays in the upper left hand corner you should see a little arrow that changes size and moves around.

A few suggestions, Darryn:-

I lowered the Speed Hold Selector to 300 KIAS and still get surges to up over 330KIAS.

1. Over 300KIAS is FAR too fast for the 737. It remains one of my FS favourites but she's an elderly lady now! The Learning Centre recommends a cruising speed of Mach 0.74 which is about 275KIAS at 35,000 feet. 'Over 300KIAS' would be around Mach 0.85 which no 737 ever built would be comfortable with! Strongly recommend using Mach speeds above 25,000 or so, anyway. Why ever not?

2. Quick way of checking wind speed/direction is to press 'Shift+Z,' which brings the figures (plus other useful info.) up in red at the top of the screen. Keep pressing 'Shift+Z' to clear.

3. Make sure you have 'pitot heat' switched on before you take off - leave alone at 35,000 feet! Otherwise the pitot head is ('virtually') certain to freeze up and start showing some really cockeyed speed variations. Maybe check the outside air temperature too, you might need engine heat as well.

It's a 747-400, not a 737.

And at FL350there is very little humidity and the probability of instant pitot tube freeze is rather small. Or you'd see airframe ice all over the place...
The pitot tube is certainly NOT certain to freeze up at FL350.

[quote](Well, before I teased you, I probably should have checked that typo in my subject line.

As it happens I've returned to jets for a while and am currently 'testing' my new 787. On my current flight I happen to be flying at Mach 0.85 at 37,000 feet, pretty similar to the flight you describe; so I did a trial. Mach 0.85 at that height is (only ) 282KIAS. So 'over 300KIAS' at cruising altitude appears to be a definite no-no in ANY airliner.

Nav,

Thanks for another great post.  Looking at the information you provided, setting the A/T to 330KIAS at FL340 must have been pushing the 747 well past its performance envelope.  If Mach 0.85 was reading as 282KIAS, I must have been telling the poor A/P to try and take the aircraft up to somewhere near Mach 1.



Oh, and speaking of not imagining things, thanks for confirming the lack of an immediately available Pitot Heat switch on the default 747 control panel.  That was driving me nuts looking for it.  I may make a couple of test flights at altitude with the Anti-Ice on and off to see if that has any effect on instrument readings.  I may also just see what the keyboard shortcut is for the Pitot Heat; probably wouldn't hurt to know.

I don't know that I have the time for another trans-oceanic flight this weekend, but I may try something across the U.S., maybe Detroit or Minneapolis-St. Paul to Seattle.  Though, I will probably use a 737 for that flight, and not a 747.  I mean, wouldn't want overkill now, would we?  Either way, at elevation, I may try switching the A/T between Mach value and IAS to see if I get any similar surges.  Be interesting to find out.

Appreciatively yours,
~Darrin

PS
Nice looking 787 you have their!  Now, if you give it back to Boeing, they might be able to get closer to finishing their testing!  ;)
~D