Transition to IAS on descent
 

Hi,

Saw a couple of old threads with regards to speed on tansition. The crossover altitude is transparent to us, what are the ways to know when Mach has changed to IAS on descent in the Boeings?

Typically, dependent upon temperature, the "crossover' altitude , at least in tropical regions occurs around 29000 feet.At this point in the atmosphere (the only point where the TAS for both mach number and the indicated airspeed is the same), your Boeing or Airbus or whatever aircraft, it doesnt matter which, will display a change from either IAS to MACH NUMBER, depending whether you are climbing or descending.

ATC like to refer to this point as the "transition" in their instructions, as you will "transition" from one speed reference to the other during, typically on descent, and if you stare at your FMS/FMC around this altitude you will spot the change over.......

Actually in the UK they refer to it as “on conversion”, to avoid confusion with the transition altitude used for altimeter setting.

what are the ways to know when Mach has changed to IAS on descent in the Boeings?

Observing the speed tape passing FL300 should give a clue. That is where you'd expect it to start to change over. As has been said there are various parameters affecting the specifics, but FL280-300 is a common window.

Yeah I've always just called it 300@30

Thanks but speed tape is always showing IAS? Unless you are referring to the Mach no below the speed tape (For Boeings)?

The command speed at the top of the tape will change from Mach to IAS.

Exactly...the commanded display speed above the speed tape tells you when it has “converted” from commanding a Mach to commanding an airspeed.

Thanks guys. Got it

For any given Mach/IAS there is only one altitude where they are equal to each other.

Thanks but speed tape is always showing IAS?

Indeed. I'm assuming you fly something like 280kts from 'conversion' to FL100. At CRZ 0.79 IAS will be about 245kts. At conversion you'll see the IAS flown increase from 245 - 280kts. This is what I meant by 'observing the speed tape'. There are some airlines that fly such a low CI that they descend at 250 after conversion. Less easy to see.

Climb/Descent Rate
 

Another thing to notice is the change in climb or descent rate when passing from following constant IAS to constant Mach and vice versa. On climb at lower altitudes when following constant IAS the airplanes true airspeed will be increasing as altitude increases. During this portion of the climb, the airplane is both increasing altitude and increasing speed. Excess thrust (that greater than drag) is being used to increase both potential and kinetic energy. Once the airplane has climbed high enough to be into the region where Mach is key, the climb will continue at constant Mach number. During this portion of the climb, true airspeed is decreasing as air temperature decreases and the speed of sound goes down. As a result, while potential energy continues to increase, kinetic energy is actually going down. There should be small, but notable increase in climb rate at the point where the climb transitions from constant IAS to constant Mach number.

The reverse occurs on the descent where the vertical speed at a given thrust setting will be higher when at higher altitude following a constant Mach line down vs. at lower altitude following constant IAS. This effect is what makes speed control going around the corner from Mach to IAS during an emergency descent following Mmo and Vmo somewhat interesting. There is a tendency to overshoot Vmo.

Please excuse the thread drift. It may be helpful to some to think about the physics of the event a little. "Know your equipment better and you are more likely to understand how it behaves - both every day and when the unexpected jumps out to get you."

From my good friend Old Smokey

"CAS / Mach Changeover Height (Given CAS and Mach Number)

Ps / P0 = (((Vc / a0)^2 / 5 + 1)^3.5 - 1) / ((1 + M^2 / 5)^3.5 - 1)

Where -

a0 = ISA Sea Level Speed of Sound (661.4787442 Kt)
Vc = CAS in Knots
M = Mach Number
P0 = ISA Sea Level Pressure (1013.25 hPa)
Ps = Static Pressure at Flight Altitude (hPa)"

Regardless of aircraft type and symbology, usually it is immediately obvious as the aircraft pitches nose up slightly and you see either a ROC increase or a ROD decrease. Often followed by the F/O moaning that the AP isn't very good at holding a steady flight path. Often followed by ones own eyes rolling upwards as a symptom of profound depression... ;)

Use your prayer wheel:

https://s26.postimg.org/6p3l2p0m1/IAS-_Mach-_Alt.jpg

Well Hallelujah! :}

Those were the days my friend, I thought they'd never end, they'd last for ever & a day........

and then along came HP with calculators & Microsoft and took all the fun out of it. It was like those who speak in strange tongues and have their secrets from the ignorant. Then along came the button pushers and everyone knew their secrets.

From my good friend Old Smokey

Ken hasn't posted in quite some time ... anyone know his present circumstances ? A long time since our paths crossed other than via PPRuNe ..

The crossover altitudes during climb and descend change on each flight each day. When using a standard CI this transition occurs often at similar altitudes.

Provided VNAV is used to climb and descend:

• CLIMB
  • VNAV works out an IAS for climb based on Cost Index, Weight, predicted Headwind Component and ISA Deviation at cruise level (based on the PERF INIT CRZ WIND and ISA DEV entered in preflight, based on track direction 180nm downstream of liftoff point magenta line);
  • VNAV works out an initial CRZ Mach number based on (again PERF INIT references are used) initial Flight Level, Weight, Headwind Component and CI;
  • VNAV changeover occurs when the CLB IAS equates to initial CRZ Mach;
  • Changeover can be observed on CLB page in FMC and above speed tape;
• DESCENT
  • VNAV works out a descent IAS based on CI (no other inputs);
  • VNAV Changeover occurs when the last used CRZ Mach matches this CI derived IAS;
  • Changeover can be observed on DESC page in FMC and above speed tape;

Provided VNAV is NOT used to climb and descend: passing FL260 an automatic changeover from IAS to Mach and vice versa occurs

J_T he's alive...I'll PM you more info about him when I get the chance because I'm not sure if he wants his circumstances made public