Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Transition to IAS on descent

Tech Log The very best in practical technical discussion on the web

Transition to IAS on descent

Old 21st Mar 2018, 04:52
  #1 (permalink)  
Thread Starter
 
Join Date: Apr 2008
Location: Test
Age: 35
Posts: 230
Likes: 0
Received 0 Likes on 0 Posts
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?
extricate is offline  
Old 21st Mar 2018, 09:21
  #2 (permalink)  
 
Join Date: Nov 2004
Location: bkk
Posts: 285
Likes: 0
Received 0 Likes on 0 Posts
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.......
piratepete is offline  
Old 21st Mar 2018, 10:56
  #3 (permalink)  
 
Join Date: May 2005
Location: France
Age: 69
Posts: 1,142
Received 3 Likes on 2 Posts
ATC like to refer to this point as the "transition" in their instructions,
Actually in the UK they refer to it as “on conversion”, to avoid confusion with the transition altitude used for altimeter setting.
eckhard is offline  
Old 21st Mar 2018, 11:01
  #4 (permalink)  
 
Join Date: Jun 2000
Location: last time I looked I was still here.
Posts: 4,507
Likes: 0
Received 0 Likes on 0 Posts
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.
RAT 5 is offline  
Old 21st Mar 2018, 11:03
  #5 (permalink)  
 
Join Date: Aug 2007
Location: sincity
Posts: 1,182
Received 17 Likes on 8 Posts
Yeah I've always just called it 300@30
maggot is offline  
Old 21st Mar 2018, 14:01
  #6 (permalink)  
Thread Starter
 
Join Date: Apr 2008
Location: Test
Age: 35
Posts: 230
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by RAT 5
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.

Thanks but speed tape is always showing IAS? Unless you are referring to the Mach no below the speed tape (For Boeings)?
extricate is offline  
Old 21st Mar 2018, 14:10
  #7 (permalink)  
 
Join Date: Nov 2007
Location: Texas
Posts: 1,918
Likes: 0
Received 1 Like on 1 Post
The command speed at the top of the tape will change from Mach to IAS.
MarkerInbound is offline  
Old 21st Mar 2018, 14:22
  #8 (permalink)  
 
Join Date: Feb 2001
Location: The Winchester
Posts: 6,520
Received 5 Likes on 5 Posts
Originally Posted by MarkerInbound
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.
wiggy is offline  
Old 21st Mar 2018, 15:08
  #9 (permalink)  
Thread Starter
 
Join Date: Apr 2008
Location: Test
Age: 35
Posts: 230
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by MarkerInbound
The command speed at the top of the tape will change from Mach to IAS.

Thanks guys. Got it
extricate is offline  
Old 21st Mar 2018, 16:27
  #10 (permalink)  
 
Join Date: Sep 2004
Location: Yorkshire
Posts: 607
Received 10 Likes on 7 Posts
For any given Mach/IAS there is only one altitude where they are equal to each other.
H Peacock is offline  
Old 21st Mar 2018, 20:58
  #11 (permalink)  
 
Join Date: Jun 2000
Location: last time I looked I was still here.
Posts: 4,507
Likes: 0
Received 0 Likes on 0 Posts
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.
RAT 5 is offline  
Old 22nd Mar 2018, 18:51
  #12 (permalink)  
 
Join Date: Feb 2009
Location: Seattle
Posts: 379
Likes: 0
Received 0 Likes on 0 Posts
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."
FCeng84 is offline  
Old 22nd Mar 2018, 20:12
  #13 (permalink)  
 
Join Date: Dec 2006
Location: The No Transgression Zone
Posts: 2,483
Received 5 Likes on 3 Posts
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)"
Pugilistic Animus is offline  
Old 23rd Mar 2018, 12:04
  #14 (permalink)  
 
Join Date: Aug 1999
Location: England
Posts: 1,050
Likes: 0
Received 0 Likes on 0 Posts
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...
Capt Pit Bull is offline  
Old 23rd Mar 2018, 16:01
  #15 (permalink)  
 
Join Date: Mar 2002
Location: Seat 1A
Posts: 8,509
Received 58 Likes on 37 Posts
Use your prayer wheel:

Capn Bloggs is offline  
Old 24th Mar 2018, 02:03
  #16 (permalink)  
 
Join Date: Dec 2006
Location: The No Transgression Zone
Posts: 2,483
Received 5 Likes on 3 Posts
Well Hallelujah!
Pugilistic Animus is offline  
Old 24th Mar 2018, 09:36
  #17 (permalink)  
 
Join Date: Jun 2000
Location: last time I looked I was still here.
Posts: 4,507
Likes: 0
Received 0 Likes on 0 Posts
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.
RAT 5 is offline  
Old 24th Mar 2018, 10:03
  #18 (permalink)  
Moderator
 
Join Date: Apr 2001
Location: various places .....
Posts: 7,169
Received 89 Likes on 60 Posts
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 ..
john_tullamarine is online now  
Old 24th Mar 2018, 12:06
  #19 (permalink)  
 
Join Date: Jan 2000
Location: FL410
Posts: 860
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by extricate
The crossover altitude is transparent to us, what are the ways to know when Mach has changed to IAS on descent in the Boeings?
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
Skyjob is offline  
Old 24th Mar 2018, 13:51
  #20 (permalink)  
 
Join Date: Dec 2006
Location: The No Transgression Zone
Posts: 2,483
Received 5 Likes on 3 Posts
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
Pugilistic Animus is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.