Jet Cruising Speeds.....

[tennm1980]

ok--im aware that most large jets cruise around 500+, for some reason, even after leveling off on autopilot, the speed never seems to increase beyond 370. also, once i get above 335, the overspeed warning starts and doesnt stop until i remain under 335. i use a saitek flight control system, and the throttle is wide open but never seems to gain good speed. am i missing something? thnx

[WebbPA]

Yes, large jets cruise at about 300 knots indicated airspeed, 600 knots or so groundspeed.

The critical factor for the pilot is airspeed.

[Ecko]

Yeah, the 500 knots youre talking about is 'the ture airspeed' also called 'groundspeed'. The 335 is the indicated airspeed.

Wich means that even though youre cruising at 600 knots true air speed, the airplane will perform as if it is flying 335 knots.

[tennm1980]

ugghhh...lol---ok---so the speed indicated on my instrument panel is correct then? it should NOT say 500 then? lol--sorry im a bit confused--if you just tell me that the 335 is good then ill have my answer ;-)  thnx

[Carlo]

Try this. Fly at cruise level say 20-25000 feet. Fly at an Indicated Airspeed of 335 Knts. Now open your GPS and have a look at the bottomleft number. Thats your Groundspeed. See the difference? Hope this satisfies your question.

Crash

PS. At these altitudes it's easier to use Machnumbers. It's less confusing.

C.

[dave3cu]

no expert here, but once your at altitude, and registering your 330kias, bring up your gps and check the ground speed to. You'll be pleasantly surprised.

Also, and I'm not sure at what flight level, but dont they switch to

[exnihilo]

The reason your IAS (indicated airspeed) never gets up all that high is due to the air pressure at high altitude being much less than the pressure at low altitude.  IAS is measured using a device called the "pitot tube", which is sort of like a hollow straw with a pressure sensor at the bottom, pointed into the oncoming airstream.  Since the ambient air pressure at high altitude is greatly reduced compared to lower altitudes, the IAS will naturally tend to be lower the higher you go.

The TAS (true airspeed), and ground speed are derived from the IAS.  All that really matters for the purpose of flying the plane is the IAS, as this is really just a measure of the dynamic pressure on the airplane caused by the forward speed.  Lift and controllability are governed by dynamic pressure.

So at sea level, IAS is equal to TAS.  The groundspeed is the same as well, once the local windspeed is taken into account.  As the aircraft climbs, if the IAS remains constant, the TAS and groundspeed both increase, until up at cruise level (30K or more feet for a conventional airliner), the TAS (and groundspeed) are quite a bit higher than the IAS.  Viola, the TAS can then reach 500+ mph.

This also means that aircraft simply cannot go all that fast at low altitude, and are not nearly as fuel efficient at lower altitudes.

About mach number.  Mach number is somewhat influenced by pressure, yes.  However, temperature has a surprisingly strong effect on mach number.  Controllability is of course highly dependent on mach as well, and if you notice, many cockpit speed indicators have a "mach" option (and autopilots have a mach option as well).

Complicated enough for you?

[Boomtown Rat]

I used to have that problem, and I thought the only way to solve it was to fly lower.

[TwoLow]

I just learned a lot!

My dad tried to explain the pitot tube to me but I didn't understand till now. He has a overly complex way of explaining things.

If the pitot heat isn't used and the tube freezes up does it actually affect the air speed gauge in the game?

[hatter]

In other words, yes, 300 some KIAS is good.

[beefhole]

If the pitot heat isn't used and the tube freezes up does it actually affect the air speed gauge in the game?

It's happened to me before-twice

Don't worry about your KIAS in cruise, it really doesn't matter-only your Mach and GS really matter at altitude.  Don't cruise based on KIAS.

[t_alexander21]

Pretty much what everyone has said above is exactly right. Once you cross 18,000 you might want to switch over to mach hold. To answer your question more directly most commercial jets cruise between .78 and .82 mach but it really depends on the type

[jrpilot]

It depends on many factors.  Onother one is the cost index.

[beefhole]

Pretty much what everyone has said above is exactly right. Once you cross 18,000 you might want to switch over to mach hold. To answer your question more directly most commercial jets cruise between .78 and .82 mach but it really depends on the type

For most aircraft, if you climb them correctly, you should probably just leave it at climb speed (280 kts for two engines, around 290 for three, 310 for four engines) until you approach within .02 of your desired mach, which usually won't happen until you're either very close to or at your crusing altitude.

To maintain your climb speed, you will need to progressively decrease your VS as you gain in altitude.  Your N2 will be the limiting factor here, adjust your VS so it doesn't go into the red too much