Hi all-again.Could somebody please tell me,a.Why is it important to switch the ISA to MACH in the jets and at what altitude should you do that?b.Why does the pressure needs to be set to 'Stnd' at transition altitude,and whats the difference between 'trans. level' and 'trans. altitude'c. Whats decision altitude-and why do you need to set the 'baro' to that level.I do realise that these may be very trivial and basic questions-I apologise for wasteing soemones time.I have tried looking them up and the answers don't make much sense.Looking forward to your help.Qas

>a.Why is it important to switch the ISA to MACH in the jets>and at what > altitude should you do that?Because most jet transport aircraft have both a Vmo (maximum structural airspeed) and Vmmo (maximum mach number). Vmo is specified to prevent exceeding maximum allowable air loads on the aircraft structure, where Vmmo is typically specified to avoid exceeding a mach number where supersonic airflow over parts of the aircraft causes undesireable effects such as shifts in the center of lift, shock-wave blanking of control surfaces, turbine blade stalls due to shock-wave effects etc etc. Mach tuck, a potentially uncontrollable nose-down tendency, is one of the more serious effects of exceeding Vmmo in most transport jet aircraft. Vmo is generally more limiting at higher temperatures and densities (IOW lower altitudes) and mach at lower temp/density (higher altitudes)Typical changeover occurs at that point in a climb/descent when the climb/descent profile airspeed equals the climb/descent profile mach number. So for a B737 with a 280 KIAS/M0.70 climb profile, you'd fly 280 KIAS until the point where 280KIAS = M0.70, then continue to climb at M0.70. On descent it's the opposite...descend at M0.74 until 300KIAS = M0.74, then 300KIAS. This changeover typically happens in the neighborhood of FL290 in most transport category jets.>b.Why does the pressure needs to be set to 'Stnd' at>transition > altitude,and whats the difference between 'trans. level' and>> 'trans. altitude'Because accurate air pressure readings at high altitudes are not commonly available (especially in oceanic airspace), as they are at lower levels due to terrestrial reporting stations. So everyone uses a standard common setting to ensure separation from each other. Down low, it's also important to ensure separation between aircraft and cumulogranite clouds, so altitudes must be standardized with respect to the terrain and not just the other aircraft in the area.Transition altitude is the altitude at which the change to STD press occurs in a climb...all altitudes above the TA are flight levels. Transition level is the flight level at which the change to QNH/QFE occurs in a descent.>c. Whats decision altitude-and why do you need to set the>'baro'> to that level.There are two types of approach altitude minima, neither of which is called "decision altitude." Decision Height (DH) is the lowest altitude at which the decision to land or go-around can be made while on a precision approach (one that has both course and glidepath guidance). For a Category I ILS, MLS, or Precision Approach Radar, the DH is a barometric altitude. For Category II and III ILS approaches the DH will be a radar altitude.The Minimum Descent Altitude (MDA) is the lowest allowable altitude on the final segment of a nonprecision approach (course guidance without glidepath guidance). It is always barometric.RegardsBob ScottATP IMEL Gulfstream II-III-IV-VSantiago de Chile

Hi BobMany thanks for taking time help with such a comprehensive explanation.qas

>Transition level is the flight level at which the change to QNH/QFE occurs in a descent.Transition Level is the first available flight level above the Transition Altitude that will ensure safe vertical separation at different values of regional QNH.Taking a Transition Altitude of 3000 ft (based on a typical UK value) then if the QNH is less than 1013 mb, FL35 would be less than 3500 ft. This means there would be less than 500 ft separation between a Transition Altitude of 3000 ft and FL35. In this case, the Transition Level would be FL40 to achieve 500 ft or better separation.

hello folks i too would like to know the first question as to why you go to mach in jets , im not understanding you fully.i never truly understood that, are you saying that in mach the plane flys differantly.im sorry for my ignorance.......Robert

So how do you calculate TA and TL?

Good questions and a great answer, Bob! Also - here's a good pictorial site on basic "mach":-http://www.grc.nasa.gov/WWW/K-12/airplane/mach.htmlRegards,

Robert, I think your question is mostly related to air density.Why do we want to fly at altitudes of 20 and 30 thousand feet? It's because the airframe can go faster up there, ground speed wise, at those altitudes because the air is thinner, less dense up there.If your body could feel the same air pressures the airframe does it wouldn't know if it was flying at 7000 feet or 27000 feet at a constant airspeed. The big advantage is a higher ground speed in the less dense air. Prop planes can make the change to reading airspeed by mach numbers the same as the jets do, around 18000 feet because of air density. The same air density changes that require you to set the altimeter to standard pressure. If you have the aeroworx B200 you can see the barber pole (never exceed speed) on the airspeed indicator move lower on the dial as the altitude increases. What you are seeing is less impact pressure on the airspeed indicator because of less air density. It's the same air pressure stresses that are impacting the airframe. Propeller efficiency in level flight I don't think would allow the sound barrier to be broken but most jets can do it in level flight. The early Lear jets had a coffin corner of around 10 miles per hour if I remember correctly. At cruise altitude, 10 miles per hour separated you from stall and a high speed tuck. The airplane wasn't certified to fly at those altitudes unless you had a working autopilot. Mach number is a ratio of the speed of sound at sea level to the speed at altitude. We are concerned about this because of the formation of the shock waves, both on the wings and turbine engine inlets. It's the shockwave that can change how the airplane flies and the mach number is a way of measuring when it can form in air that is less dense than at sea level.I hope I haven't contributed to your confusion.Daryll

i believe i have a good grasp on it , but the big question is say im on the runway and i take off, i take off with the groung speed of 150 when do i change over to mach, above 18000 and if i do and i see say .78 in the mach window why is that information important to me.i guess what im getting at is if im at 36000 ft and im flying at 500 miles an hour why is it important to see say .80 mach in the window what is that doing for me as for information.im so sorry for my ignorance i do believe what mach means i just dont know why i have to know and see that in the speed gauge. so if im going .80 and i decide to change over and see my speed what does that do for me.i will read some more on that info............Robert

Daryll has it. The higher you fly, the lower the air pressure. Lower air pressure means a less dense volume of air to measure. So if the air is thinner, there would be less indication using the standard pitot tube type measurement of airspeed. I think that 500 knots IAS at FL180 would be much faster than 500 knots TAS and it gets more pronounced at higher altitudes as in class A airspace, say FL350. You probably don't have enough engine to make your airspeed indicator reach 500 knots IAS at FL350.Dolph