Airspeed. Why so many?

B767Driver

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.
 
The only thing I would add is that the reason IAS dropps off with altitude is because of the difference in the density of the air aloft, since IAS works off of ram air.

If you are looking for a good book, if I remember right, Flight Theory for Pilots by Cole is a good resource. I'll have to check my books at the airport tomorrow to verify the name, author
 
B767Driver said:
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.
Click to expand...

I guess you've been around longer than me, but...do most civilians really not know this stuff?

I was taught the differences and reasons for IAS vs. TAS as a private pilot, and the reason for the mach meter during my commercial training. I thought it was common knowledge. I don't know the nitty gritty technical ways to calculate them, but I have a working knowledge of the differences between them and why it matters. I'm surprised somebody could make it to the level of CFI without having the same understanding.
 
You may have been taught it...but unless you're up there using it and seeing it...I can see how the lack of application could lead to lack of understanding. I don't see it as undermining his CFI abilities.

What if you only studied crosswind landings...but never did one. Would you have the same level of understanding of the application of the maneuver.
 
B767Driver said:
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. .
Click to expand...
Maybe where you civilian guys fly, but my home base is at almost 6000 msl. Add a summer day and that means 8,000- 10,000 msl.

That's enough change in TAS from IAS to have caused more than one takeoff and landing accident in this area because the pilot didn't understand what the differences were.
 
I understand. Good point. You have the privilege to observe first hand the effects everyday. My intent, however, was to steer the discussion toward the difference in high altitude/high speed flight, where the difference in meters can be a couple of hundred knots or more, as opposed to low altitude/low speed flight where density can cause a change of 20 kts or so.
My point was that, I'm not sure too many low altitude fliers realize the drastic differences in airspeed meters at high altitude, presumably, however, most would understand the significance each possesses.
 
There's also groundspeed that you get from TAS when computing for time enroute. I used to be confused about those two very early on cause I really thought they were the same, even when doing the planning without knowing;big mistake. If TAS increases for a given wind velocity, then it'll affect the plane less since it needs a smaller WCA to trace the groundtrack. There's less time enroute. But overall the groundspeed would still be the same(when compared to a lower TAS) since the wind velocity has not changed.
 
B767Driver said:
You may have been taught it...but unless you're up there using it and seeing it...I can see how the lack of application could lead to lack of understanding. I don't see it as undermining his CFI abilities.
Click to expand...

No, I'm not saying he's a bad CFI. I know how easy it is to get rusty. But I still think CFI's ought to know this stuff. It's just another concept to be taught, like density altitude, or adverse yaw, or whatever.

The whole IAS vs. TAS discussion usually comes up for me when I'm going over XC planning with a private student. The aircraft performance charts are marked in TAS, not IAS, so it's a good time to talk about it.

If I fly with a commercial student in our Arrow, I can even show them the difference in the air. Our Arrow has one of those nifty airspeed indicators with the rotating wheel that allows you to calibrate it for TAS while flying. Up at 8000 or 10,000 the difference is quite noticeable.
 
Here's an application example.

From 10,000 MSL you maintain 300 kIAS until you reach .80 Mach. During this time is your speed remaining the same? Well yes and no. IAS is obviously remaining the same. But TAS and Mach are increasing at a pretty good rate. So while your IAS is steady at 300, the speed over the ground is picking up (assuming no wind).

What would happen if you reach .80 Mach at FL270 but continue to maintain 300 KIAS in the climb to FL410? The Mach would continue to increase...you'll probably feel some buzz in the flight controls indicating you entered the transonic speed range. So now transition to the machmeter to keep the thing subsonic...and let the IAS drop in favor of Mach. (I've bought the beers before not paying attention to crossover mach. Yes, Non FMS airplane!)

When we're flying our C152s, Barons and King Airs...even though we know the principles of each speed...there's just not a lot of practical application until you get to see it in a jet. Then it makes perfect sense. Until then...I can see how it would be a bit nebulous.
 
I flew a C-421 with an airspeed indicator that had an automatic TAS dial in the middle of the gauge. That really put a solid understanding of what I'd been taught about airspeeds
 
Guys and gals,

Take it easy on B767. From what I've seen, he's a very experienced jet driver trying to impart knowledge on us flying in the lower altitudes about high-speed, high altitude driving. I know we are all supposed to know this stuff, but keep in mind not all of us remember how to calculate mach number, see it in action, etc. I realize most were probably not criticizing him, but it definitely came off as being holier than thou with some of the remarks. Just remember, not everybody is a perfect CFI. Not trying to criticize anybody or single anybody out...

B767 keep the good info coming.

TX (hopefully only 4-6 months from Lear upgrade to be in those FL's)
 
txpilot said:
Guys and gals,

Take it easy on B767. From what I've seen, he's a very experienced jet driver trying to impart knowledge on us flying in the lower altitudes about high-speed, high altitude driving. I know we are all supposed to know this stuff, but keep in mind not all of us remember how to calculate mach number, see it in action, etc. I realize most were probably not criticizing him, but it definitely came off as being holier than thou with some of the remarks. Just remember, not everybody is a perfect CFI. Not trying to criticize anybody or single anybody out...

B767 keep the good info coming.

TX (hopefully only 4-6 months from Lear upgrade to be in those FL's)
Click to expand...

Thanks. I'm not too proud to admit I wasn't too adept at high altitude flying back in the day. After several thousand hours beating around in Barons below 5000' and flying commuters (when they had props on the engines and did not go above FL250)...I didn't really know jack about jet climb and cruise performance. The concept of climbing at an IAS to a Mach crossover point didn't have much relevance to what I was doing.

As a matter of fact, transitioning from a Part 23 certified airplane to a Part 25 certified one was quite a paradigm shift just in climb performance alone...leaving behind blue line and Vyse for V2 speeds and 1st, 2nd and 3rd stage climb segments...and then the approach climb and landing climb performance requirements as well.

When you upgrade to the Learjet, it will be a great feeling knowing that you can pull it up to a climb attitude and know that if you lose an engine you just lower the nose a bit and keep on trucking...and not have to worry about what field you might have to land in (yes...even in a light twin). On every Baron takeoff I always surveyed the area to determine where I was going to land if the engine quit...because I always had so much freight in the back...not much chance it was going to climb after takeoff if the cylinders blew. In the Lear...you know it's going to climb.
 
Yes, it'll be nice. I've actually spent more time than I ever thought I would sideways in our Lears (our jumpseats sit sideways, of course) so I've seen a lot more than most of our pilots on normal routes as far as how they operate the Lears.

As far as Barons, I'll take a Baron over a Navajo, both fully loaded any day, especially with an engine failure. At least the Baron will only be (hopefully) -50 to -100 fpm. And I won't even bring up the Caravan on engine failure ;) . Did somebody say popsicle in icing :eek: ?
 
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