B767Driver
New Member
I was at the airport yesterday talking with a C172 instructor and we got to talking about airspeed. With some of the questions he was asking...it dawned on me...that there is really not much training on or material written about airspeeds and high altitude flight for us civilian guys. The military guys are well versed in this area because they have aircraft capable for such flight and do it in their primary training. For us civilian guys...we're lacking.
So what's the deal with Indicated Airspeed, True Airspeed and Mach Number? And how does a pilot use each in the Flight Levels?
Below 5000' MSL (where us civilian guys learn to fly) there is just not much difference between what you see on your IAS and the TAS. Basically, what you see on your airspeed indicator is a fair representation for how fast you are going. Fast climb to FL370. The IAS has very little relationship to how fast the airplane is moving through the air. As you climb...the IAS will drop but the TAS will increase. For example...your airspeed indicator is showing 250 kias...but the TAS is 475kts!
For time and distance and flight planning calculations, the IAS is worthless. Therefore, TAS will be used to determine time enroute, fuel requirements, etc. It is a good representation for how fast the airplane is truly travelling.
However, the IAS is very valuable from a stick and rudder standpoint. It continues to measure the pressure of air flowing over the wings...and this is important from an aerodynamics standpoint. The wing will still stall at the same IAS at FL370 as it will at 5000' (for all practicable purposes). So even though the IAS is not a worthwhile value for flight planning purposes at high altitude...it is still valuable in preventing the airplane from stalling and the airframe from overspeeding. At FL 370 you can have a TAS of 350 and stall the airplane at 150KIAS in level flight.
So now Mach Number, where does this fit in? If TAS is a good representation of how fast you are actually flying...why not use a TAS indicator for speed adjustments instead of a Machmeter at high altitudes in the Flight Levels?
The reason for this is that at high altitude flight...the Mach Number becomes a critical factor in the cruize performance of the airplane. If the Mach Number gets too high...the transonic speed range becomes a factor and all of the aerodynamic characteristics associated with it. Using only a TAS Indicator does not give us a good picture of the relationship to the critical mach number. The Machmeter does. It keeps us from exceeding the critical mach number and gives us a good awareness of our speed in relationship to this range.
So IAS will always and everywhere assist you with flying the airplane and preventing it from stalling. The TAS will allow you to accurately gage your progress in relationship to time, fuel, distance and all of the flight planning factors. Mach Number is key in the Flight Levels to assist in the awareness of the transonic range and to prevent us from encroaching upon transonic flight.
I thought they were good questions.
Feel free to add more info.
So what's the deal with Indicated Airspeed, True Airspeed and Mach Number? And how does a pilot use each in the Flight Levels?
Below 5000' MSL (where us civilian guys learn to fly) there is just not much difference between what you see on your IAS and the TAS. Basically, what you see on your airspeed indicator is a fair representation for how fast you are going. Fast climb to FL370. The IAS has very little relationship to how fast the airplane is moving through the air. As you climb...the IAS will drop but the TAS will increase. For example...your airspeed indicator is showing 250 kias...but the TAS is 475kts!
For time and distance and flight planning calculations, the IAS is worthless. Therefore, TAS will be used to determine time enroute, fuel requirements, etc. It is a good representation for how fast the airplane is truly travelling.
However, the IAS is very valuable from a stick and rudder standpoint. It continues to measure the pressure of air flowing over the wings...and this is important from an aerodynamics standpoint. The wing will still stall at the same IAS at FL370 as it will at 5000' (for all practicable purposes). So even though the IAS is not a worthwhile value for flight planning purposes at high altitude...it is still valuable in preventing the airplane from stalling and the airframe from overspeeding. At FL 370 you can have a TAS of 350 and stall the airplane at 150KIAS in level flight.
So now Mach Number, where does this fit in? If TAS is a good representation of how fast you are actually flying...why not use a TAS indicator for speed adjustments instead of a Machmeter at high altitudes in the Flight Levels?
The reason for this is that at high altitude flight...the Mach Number becomes a critical factor in the cruize performance of the airplane. If the Mach Number gets too high...the transonic speed range becomes a factor and all of the aerodynamic characteristics associated with it. Using only a TAS Indicator does not give us a good picture of the relationship to the critical mach number. The Machmeter does. It keeps us from exceeding the critical mach number and gives us a good awareness of our speed in relationship to this range.
So IAS will always and everywhere assist you with flying the airplane and preventing it from stalling. The TAS will allow you to accurately gage your progress in relationship to time, fuel, distance and all of the flight planning factors. Mach Number is key in the Flight Levels to assist in the awareness of the transonic range and to prevent us from encroaching upon transonic flight.
I thought they were good questions.
Feel free to add more info.