Hi All,

 

I did a quick flight from KBNA to KGSP at FL290 in the 700 last night. During my preflight, I attempted to calculate my TAS in knots based on the flight level, Mach speed, and ISA deviation. I am having trouble estimating my mach speed. Using 120,000 pounds, and a pressure altitude of 29,000, the Long Range Cruise Control chart on page PI.21.2 said my mach should have been .675. At cruise, based on my cost index of 34, my actual mach was .728. For the purposes of using the Long Range Cruise Control chart, is pressure altitude the same as cruise altitude? Do you need to adjust the results of the charts based on the cost index? I am going through the trouble of attempting to calculate my TAS so I can enter it in my ActiveSky and SquawkBox flight plan.

 

Thanks!

 

Mike Murphy

Pressure altitude only matches actual altitude when the baro is 29.92. Above transition you switch to std so the altimeter is giving you pressure altitude, but not your true altitude.

 

You may also need to compensate for temperature, it plays a big role in air density.

CI Affects cruise speed so that would also be a factor to consider.

http://www.aero.sors.fr/adacalc.html

 

vololiberista

Unless you actually select LRC (Long Range Cruise) as the speed you want, the plane will fly at ECON. ECON is based on your variable CI. LRC is found another way (dig out the Thrust Required graphs, etc).

 

So, if you left the plane in the default ECON setting, you might not match the LRC. If you select LRC on the CRZ page, you should see something closer to the Cruise Control Chart.

Yes, I agree with Matt. You can not stay in ECON and expect to see the LRC performance figures from the Long Range Cruise Control chart in the QRH performance section.

 

If you want to see the NGX match what’s in the QRH performance section, then on the CRZ page, insert .675 in the Target Speed value. You should see very close the performance figures that were on the Long Range Cruise Control chart. But this is probably not what you want to do. Realistically, if your airline wanted to save fuel cost, they would give you a very low Cost Index or instruct you to put the FMC CRZ in LRC for the flight.

But going back to your question about finding your TAS, a real world dispatch would provide the captain with a full flight plan which would have the planned flights TAS.

It's not really the captain’s task to calculate it. But you still must know what your planned TAS or mach speed is if you want to file your IFR SIM flight plan. ATC is expecting you to fly the TAS/Mach speed on the flight plan and dispatch does too. If you don't, the flight plan won't be as accurate. Any time you adjust your IAS speed 10 knots or .02 mach, you must advise ATC. Of course, adjusting IAS will change TAS/Mach speed too.

 

To figure your TAS, use this rule of thumb; Take half of your altitude and add it to your indicated airspeed (IAS).

Example: 252 KIAS at FL290

29000/2 = 145 + 252 = 397 KTAS

 

How did you arrive at a Cost Index of 34?

btw. you can just send a plan with M most times anyway...

Thanks for your help! I use AdaCalc and have found that very helpful to calculate TAS based on mach and ISA deviation. I use a cost index of 34 because I think I read that 34 is a reasonable cost index.