Aircraft: Specs vs. FS - Re-Visited!

[Xyn_Air]

Alright!  What gives here?!?

I just checked on the performance specifications for the Boeing 747-400 on two different sites:

Boeing 737-400 @ Airliners.net

Boeing 737-400 @ Wikipedia

In both cases, the maximum speed is listed as well over 450 knots, with a bit of difference between the two.  Even the normal, long-range cruising speed is over 425 knots.

And yet, in FS2004 whenever I get a 737-400 up over 350kias or thereabouts I get an overspeed warning.

Sooo . . .

1) Is the performance values for the Boeing 737-400 that I am using in FS2004 incorrect?

2) Are the listed performance specifications from the above sites correct?

3) Would there be a reason why the overspeed warning is set well below even the normal operational parameters?

4) Is there something about the IAS that could be messing with me?  (I don't think I have flown recently in winds over 4 knots, so I don't this is the problem, but I might as well ask just in case.)

I am, once again, confused.

OK, I know I should be sticking to my Cessna's and the like, because the passenger jets are beyond my performance envelop.  But, you know, sometimes I do like to cut loose and take those big jets for a cruise.  I haven't forgotten what I am supposed to be learning, but I also haven't forgotten how to have fun when I want.  That being rambled . . .

When I fly the 737's, I have been cruising at about 300kias.  I was wondering (for the bonus round of questions), from those of you who also fly the 737's:

A) If you also fly 737's, what IAS do you normally cruise at?

B) Which model of 737 (default or add-on) do you find to be the best?

Many, many thanks in advance for answers, assistance, and random comments that make me chuckle!

~Darrin

[Nav]

Xyn-Air, Air-Speed Indicators work on air pressure. Since air gets thinner with altitude, readings in knots (or mph or kph for that matter) are just about meaningless at altitude. At say 35,000 feet, for example, '300 knots indicated' represents a real speed of something over 500 knots.

The correct method is to work on the Mach Number above say 20,000 feet. Mach speeds are computer-calculated to allow for thinner air pressure. They are expressed as decimals of the speed of sound (e.g. '0.74 Mach' is 74% of the speed of sound).

Looking over the Wiki reference you provide, the cruising speed of the 737-400 is given as 0.74 Mach. That's about right for both the real thing AND the FS version. More modern designs like the 777 and 787 can cruise at up to about 0.82 mach, and so can their FS counterparts (see the Learning Centre for more advice).

So the best solution is, at any sort of height, forget 'knots' and work in Mach numbers; for the 737, say Mach 0.65 in the climb, Mach 0.75 cruise). Below say 20,000 feet, use knots by all means - but you'll find that 250-300 knots 'inidicated' is the fastest you'll need to or should go. And, of course, revert to knots for landing.

[Xyn_Air]

Xyn-Air, Air-Speed Indicators work on air pressure. Since air gets thinner with altitude, readings in knots (or mph or kph for that matter) are just about meaningless at altitude. At say 35,000 feet, for example, '300 knots indicated' represents a real speed of something over 500 knots.

The correct method is to work on the Mach Number above say 20,000 feet. Mach speeds are computer-calculated to allow for thinner air pressure. They are expressed as decimals of the speed of sound (e.g. '0.74 Mach' is 74% of the speed of sound).

Looking over the Wiki reference you provide, the cruising speed of the 737-400 is given as 0.74 Mach. That's about right for both the real thing AND the FS version. More modern designs like the 777 and 787 can cruise at up to about 0.82 mach, and so can their FS counterparts (see the Learning Centre for more advice).

So the best solution is, at any sort of height, forget 'knots' and work in Mach numbers; for the 737, say Mach 0.65 in the climb, Mach 0.75 cruise). Below say 20,000 feet, use knots by all means - but you'll find that 250-300 knots 'inidicated' is the fastest you'll need to or should go. And, of course, revert to knots for landing.

Doh!  :-[ Doh!  :-[ Doh!  :-[

I hate it - absolutely hate it - when I forget the basics!  Yargh!  Good grief!  And now I remember reading this very information last year here in the forums about using mach numbers at high altitudes.  Sigh.

Thank you very much for that informative refresher, Nav.  I'll put this on my list of things to get tattooed on my arm for easy reference!  ;D

Abashedly,
~Darrin

[Xyn_Air]

Alright, now that it is weeks later, I am running into a similar problem all over again.  So, I decided to resurrect this thread instead of starting a new one.  I did, however, change the name a bit to reflect the more generalized problem I am experiencing.

Today I was flying a default 747-400 in FSX.  As I am trying to be more and more conscientious about how I fly, even when I am perhaps flying something far above and beyond any real-world skill I may possess (Would I even know how to start up a 747 let alone get it off the ground? Highly effing doubtful!).  That is, I follow as many of the proper procedures and checklists as is possible given the limitations of FS.

Now, I very, very specifically read over the performance specs for the Boeing 747-400.  Ye verily (saying "ye verily" is kind of fun, I just noticed), the specs say that the VMO for the 747-400 is .88 Mach.  The Turbulent Air Penetration Speed is .73 Mach.  I set the A/T and Speed Hold to .71 Mach.  And, wouldn't just know it, I got an overspeed warning.  So, I reduced speed to .70 Mach, but still had the overspeed warning.  Finally, at .69 Mach, the overspeed warning ceased, and flight continued as normal.

Now, looking back at Nav's advice, I see that Mach is for above FL200, if I understand what he is saying correctly.  So, what I am wondering is if the following is true:

1) Below FL200 (below 20,000 feet ASL), you use KIAS, because if you use Mach value you will be setting much too high a corresponding KIAS speed.

2) Above FL200 (above 20,000 feet ASL), you use Mach value, because if you use KIAS you will be setting much too high a corresponding Mach value.

Yes?  Do I get it now?  Please, tell me I get it now!

Trying to turn all my passenger liners into supersonic fighters,

Darrin

EDIT: I originally posted this article in the FS2004 forums because that is what I was using at the time.  Now, I am using FSX, but the gist of it all should be the same either way.  Thanks.

[JBaymore]

[quote][color=#000000]Now, I very, very specifically read over the performance specs for the Boeing 747-400.

[Nav]

Nearly there, Xyn_Air.

[Xyn_Air]

John and Nav,

Thank you, thank you, so many times over thank you!  Those were great explanations and exactly the information I was looking for!

Normally I fly between 260 - 270 KIAS, or between 0.60 - 0.65 Mach, when tootling around in the big boys (I must admit to having a short attention span and being addicted to excessive amounts of variety; I even enjoy flying the ultra-light trikes around!).  When taking off I have always stuck to 250 KIAS as I leave the airport.  I don't know where I got that from (tutorial somewhere, maybe?), but I am glad to see I was doing the right thing.

Approach has never been a problem because I always try to get slow enough to start putting out flaps around 20nm out or so from the airport.  By 10nm out I am at full flaps and gear down, so I am going pretty slow at that point.

But, of late I have been wondering about cruising speeds at altitude.  I think this may have come about from learning what an issue time is for passenger airlines.  While safety is THE RULE, anywhere time can be saved is money saved/earned.  So, I guess I got to the point of looking to see how fast I could reasonably push those cigar tubes around.  Since I never had really approached VMO before, I was not really consciously aware of the issues air density, KIAS, and Mach speed would create as the aircraft climbs and descends.

Well, just goes to show how far real flying knowledge versus able to get a plane up in FS knowledge are apart!  I thank you, again, for the valuable information!  I am devouring everything up - both real and simulated - and it makes me excited to learn more!

You guys rock!  You are more than welcome to bring me up to speed anytime!

Darrin

[Brett_Henderson]

How airspeed indicators, and altimeters work, is interesting stuff.

The indicated vs true, airspeed relationship, always made sense... But I never fully understood why there was a cut-off altitude where mach became the standard.

Same for altimeters and barometric pressure. I understand that an altimeter is nothing more than a barometer, but never really understood why there was a cut-off altitude where atmospheric conditions were ignored and an altimeter setting of 29.92 was used.

Mach over FL200 ? Altimeter setting of 29.92 over FL180 ? .... Why

[Nav]

Great, Xyn_Air!

About speeds, I find that it adds interest in the end to make a formal point-to-point flightplan and then try to stick to the speeds and times it specifies; that's what the real airliner guys have to put up with! Sometimes you simply can't, of course - headwinds in particular can be too strong for even a jet to cope with; but at least the experience will make you more forgiving next time you are the victim of a late arrival!

Another thing you might try is one of the lessons - I think it's called 'Jet Pilot Checkout' or something similar. It's hard to 'pass,' as it involves taking a 737 off, flying it around the Seattle region at specific heights, speeds, rates of climb etc. for twenty minutes strictly as instructed, and then landing it - and it all has to be done manually!