Sign in to follow this  
rwy12

Speed reduction at higher altitudes

Recommended Posts

Hi EveryoneCan soembody please explain to me the following:Why is flying at high altitudes (39000ft etc) regarded more economical.I normal stick at 30000ft in the pmdg 744 and can manitain air speeds around 315When I try at 39000ft the air speed falls to around 280.The planes nose is slightly raised-I think due to lower amounts of air.So why would flying at higher altitudes be more efficient.Looking forward to your help.Qas

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Hi Qas,Your speed is greater at higher alititude, but because the air is less dense the indicated airspeed is lower. IAS is what you see inside the aircraft as that's what's critical to things like stall and overspeed conditions, it's effectively a measure of the volume of airflow over the wings: too much and you break stuff, too little and you cannot sustain level flight. On your PMDG you will see that somewhere, probably on the ND, groundspeed is displayed. If you watch it in zero wind conditions you will see that 250 knots IAS at 39,000 feet gives a much higher groundspeed than 250 knots at 10,000 feet.Hope this helps.

Share this post


Link to post
Share on other sites

The basic true airspeed (TAS) vs. indicated airspeed (IAS) question regarding thinner air at higher altitudes has been expressed.There is another factor called the Mach effect which has to do with airflow compression ahead of the aircraft at higher altitudes and airspeeds. To correct for this speed is read as Mach on your PFD starting at about FL240 and your autothrottle speed display is set as a decimal value (i.e. M.78) as displayed below the speed tape on your PFD. The operations manuals give specifications in Mach terms at high altitudes including a never exceed Mach value. The Mach value compensates for this additional compression and generally thinner average air density.FYI:There is a minimum speed stall that increases as you climb and a high speed buffet limit that decreases as you climb. If you climb to an altitude that doesn't leave much space between the two limits and then go higher, you can get into a no-recovery situation known as coffin corner, referred to in one of the linked discussions posted above. The high speed buffet limit is in terms of Mach, BTW. I'm rusty on this and it probably needs some correction. There is also the case where the relative angle of the aircraft wing engine intake in relation to the airflow direction past it can get so large that air can no longer flow into the intake causing what is known as a flameout, yet another limitation.For your reference M1.00 is the speed of sound. Some aircraft (particularly military) have been designed to go through this compression barrier withstanding temporary buffet I believe and pounding out the famous speed barrier shotgun sound to those at slower speeds below this. The Concorde was a supersonic civilian aircraft designed to go above M 1.00.Generally, when you start to reach FL240 start to pay attention to Mach speeds. Your MCP I believe will change your AT reading mode appropriately but your airspeed tape shows both. Your ND will still show the correct ground speed as this is derived from changes in navigation position using several references.It is pretty complicated and I hope I got this right but I want to expand on the resource links provided.

Share this post


Link to post
Share on other sites

In a good model, it sure it. You'll see the red bricks getting closer as you climb.* Orest

Share this post


Link to post
Share on other sites

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