IAS and Mach for climbout cruise...
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Join Date: Feb 2006
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IAS and Mach for climbout cruise...
QF and DJ Drivers,
Can anyone explain what the airlines SOPs are regarding climb on IAS and when to change over to Mach for remainder of climb and into the cruise?..
Can anyone explain what the airlines SOPs are regarding climb on IAS and when to change over to Mach for remainder of climb and into the cruise?..
Nunc est bibendum
You fly the IAS until it translates into the respective cruise mach number. A 'normal' ECON cruise climb speed in the 767 (cost index 40 which is the 'norm') at 'normal' domestic weights will result in a climb speed of 290-310 knots into .78-.79. So you climb at (say) 310 IAS until that becomes .79 (about FL 270-280) and then climb at .79.
Same on descent. You descend at CRZ mach number until that translates into a descent speed- 280 knots normally for Australian ops. So if you're really high .79 may be less than 250 knots IAS. As you descend, the IAS will increase and at 280 knots you give the mach number away and fly the IAS of 280.
Does that make sense? Happy to clarify if it doesn't.
Same on descent. You descend at CRZ mach number until that translates into a descent speed- 280 knots normally for Australian ops. So if you're really high .79 may be less than 250 knots IAS. As you descend, the IAS will increase and at 280 knots you give the mach number away and fly the IAS of 280.
Does that make sense? Happy to clarify if it doesn't.
Nunc est bibendum
If we're not in a rush. 
Again, a 'normal' cruise will see us between .78 (light weight) and .8 (heavier). This assumes cruising close to optimum flight levels also.
If we're in a hurry you'll see us doing .82-.83. If we're in a real hurry you'll see us at FL280 doing 340 IAS!
Again, a 'normal' cruise will see us between .78 (light weight) and .8 (heavier). This assumes cruising close to optimum flight levels also.
If we're in a hurry you'll see us doing .82-.83. If we're in a real hurry you'll see us at FL280 doing 340 IAS!
Join Date: Apr 2008
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Err Keg,
What do you mean by "translates"???? In your first post you twice used this term, and once the term "becomes".
What do you mean by "translates"???? In your first post you twice used this term, and once the term "becomes".
You fly the IAS until it translates into the respective cruise mach number.
I think he means the following:
Climb at a constant IAS (mach number increases) until the mach number reaches the desired number. Once this happens, climb at a constant mach number (IAS decreases) until desired cruising level reached.
Climb at a constant IAS (mach number increases) until the mach number reaches the desired number. Once this happens, climb at a constant mach number (IAS decreases) until desired cruising level reached.
Nunc est bibendum
Just to follow up on FGD135's query, 330 knots IAS at sea level is about mach 0.6 (going from memory here). 330 knots at FL 280 is about mach .81. 330 knots at FL 350 is going to be mach 1. something. So as NZScion points out, we climb at the desired IAS until it turns into/ translates to/ becomes the mach number that we intend climbing at.
At that point, as we climb at that constant mach number, our IAS will decrease. IE .79 at FL 300 is about 330 knots IAS. .79 at FL390 is about 240 knots IAS. (OAT plays a part in all of this as well but you get the general idea).
At that point, as we climb at that constant mach number, our IAS will decrease. IE .79 at FL 300 is about 330 knots IAS. .79 at FL390 is about 240 knots IAS. (OAT plays a part in all of this as well but you get the general idea).
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Keg,
I think you've explained this well, except the bit about OAT.
In fact, OAT has nothing to do with the relationship between IAS and Mach at a given FL. It probably isn't obvious on your 767, since your Econ climb/cruise is slightly different every day, and therefore the crossover FL is probably also different.
On simpler aircraft which don't have an Econ function in the FMS, which for example climb at a fixed 250KIAS/M0.72, the crossover (when 250KIAS equals M0.72) ALWAYS occurs at FL336, regardless of ISA deviation.
This stumps many since they are used to the variation in TAS for a given IAS when the ISA changes.
Cheers,
VI
I think you've explained this well, except the bit about OAT.
In fact, OAT has nothing to do with the relationship between IAS and Mach at a given FL. It probably isn't obvious on your 767, since your Econ climb/cruise is slightly different every day, and therefore the crossover FL is probably also different.
On simpler aircraft which don't have an Econ function in the FMS, which for example climb at a fixed 250KIAS/M0.72, the crossover (when 250KIAS equals M0.72) ALWAYS occurs at FL336, regardless of ISA deviation.
This stumps many since they are used to the variation in TAS for a given IAS when the ISA changes.
Cheers,
VI
