Advanced Pilot Programs

Living with "I.S.A." and the "Three Cruises"

by Ed Ward, Jr.

Last Updated: March 14, 2006


Hello everyone and "Welcome Back"!

Today we are going to talk about high altitude flying and three ways to fly an airliner in cruise flight. First, let me explain some factors that affect high altitude flight.

One factor is the weight of the airliner. The Basic Operating Weight of the airliner is given in the weight and balance documents specific to a particular airliner. Next, we want as many passengers as seats, but the airlines know (through statistical analysis) how many seats will probably be filled on a particular flight. Adding the passengers gives us the "zero fuel" weight. Based on that knowledge, the only other factor of weight is how much fuel to put in the tanks. We need enough fuel to meet the Federal Air Regulations. Airlines pay the same for contract fuel wherever they land. More than enough fuel means extra weight and less performance. Most good airlines have an allowance for the Captain's wishes. He is the "man in charge" and if he wants more fuel for some reason (like weather problems) then his wishes are granted. When the crew assembles prior to departure, the airline dispatch office will present the weather, weight and balance and computerized flight plan for the flight. The manifest is presented to the crew just before departure. At that time the crew will review these documents and recheck them.

Another thing that will affect high altitude flight is the temperature of the air. In the Flight Level Altitudes the temperature is very important. This temperature is based on "International Standard Atmosphere (ISA). I have copied a few lines of ISA below so that you can "get the drift".

hPa (mbar) ......... height (metres) ......... height (feet) ......... temperature (degC)

... (226/ISA TROP.. 11 000 ..................36 091 ................. -56.5)
.... 250 ................... 10 363 .................. 33 999 ................. -52.3
.... 300 ..................... 9 164 .................. 30 065 ................. -44.5

Now, let me point out that the above chart goes from 30,065 feet up to 36,091 feet. In the US we use feet as height and temperature in degrees centigrade (metric). OK, what I want to point out is that ISA (temperature) will be the following (I have rounded off for convince):

30,065 feet = Temperature -45 degrees C

33,999 feet = Temperature -52 C

36,091 feet = Temperature -57C

Now we have a "Standard" for reference. If, on a particular day, the temperature were warmer or colder than "Standard", the performance of the airliner would be affected. Warmer than "Standard" temperatures result in decreased performance. Temperatures colder than "Standard" result in increased performance. On a real flight, you would get the ISA difference for your current altitude right out of your airliner's FMS (flight management system). It would show up as ISA plus or minus the number of degrees difference (example, ISA + 10C, or ISA -10C etc)

.There is another way to determine what ISA is for your altitude. This method is accurate within a degree or two. For airline pilots this "rule of thumb" is most useful to remember.

Say we were flying at FL330. What is ISA (don't look at the chart)?

Here’s the formula;

Take the altitude in thousands of feet (as a negative) multiplied by 2 and subtract 15.

Using this formula, ISA at FL330 is approx. -51 degrees C. {-33X2= -66 and -66–15 = -51}

Let's do another one;

If we are flying at FL370, what is ISA?

Take the altitude in thousands (as a negative) and multiply it by two (-37X2=-74) subtract 15 and ISA for FL370 is roughly -59 degrees C.

In all cases you double the altitude and subtract 15 to give you ISA.

Say you are flying at FL330 and you see in your FMS the following:

"ISA +10C"

What is the temperature outside and what can we expect in performance? Ok, using the "rule of thumb" we again double the altitude and subtract 15. This would give us ISA of -51C. Now the FMS shows that we are plus 10C. So we take -51C and add to it a +10C. This would give us an actual temperature of -41C (caution, watch out for the math here). What this would mean is that the temperature is warmer then Standard and we would have a decrease in airline performance.

Ok, let's say we see in the FMS


Again we double the altitude and subtract 15 and come up with an ISA of -51C. Now we have to add to this -10C, which would give us a temperature of -61C. This indicates a temperature colder than Standard and it would give us better airliner performance.

One last thing, ISA is important for "step climbing" your airliner to higher altitudes. What you do is check any deviation from ISA (from your FMS) and then refer to your climb charts to see if you have the reduced weight to climb to the higher altitude. If you are ISA Plus you might have to wait and burn off more fuel before you can start your climb.

Got all that? Let's jump in MS Flight Simulator.

Using FS2000 the first thing I want you to do is in your "Pull Down Menu" go to "Options" then "Settings" then "International". Under the block "Units of Measure" select "Metric (Altimeter feet)". Once you select this then leave it (don't change it back later). Please note that I am using FS2000. If you are using FS98 or an earlier edition you need to modify this set-up accordingly.

FS2000 gives us an "across the board" temperature of ISA at all altitudes. It is set in the Flight Simulator program. However, we can see the affect of adding or subtracting temperature from Standard in airliner performance.

What I want you to do is get into your favorite WestWind Airliner and (heading east) climb to FL330. When you get there I want you to set 96 percent N1 and let the airliner "settle down". After it has "settled down", note the Mach number. What we are going to do is adjust ISA plus and minus and see what happens to performance.

Once you are at your cruise altitude (FL330) I want you to go to your "Pull Down Menu" then go to "World" then "Weather" then "Advanced Weather" then "Temp/Pressure" then set the "Altitude" to 33,000 and the "Daytime Temperature" to -51C. "Check out" of the box. You should see very little change because we have set ISA for this altitude

Now I want you to go back to "World" then "Weather" then "Advance Weather" then "Temp/Pressure". Leave the "Altitude" alone (33,000) but change the "Daytime Temperature" to -36 degrees C (ISA plus 15). Now check out of the "box" and you should see a marked decrease in your airliner's performance. The Mach should be lower due to reduced performance.

Ok, this time I want you to go back to your "Pull Down" Menu and leaving the altitude alone (33,000) this time, set the "Daytime Temperature to -66 degrees C (ISA minus 15) and check the performance of the airliner. We should see an increase in our original Mach number due to the fact that we are flying in temperatures that are colder than ISA and therefore would get increased performance.

A good question is; "What would cause air to become warmer or colder than Standard?" My guess is that it has to do with some area of "global warming". I will have to defer to a meteorologist to correctly answer this question. I will say this; In my last years of flying jets, it seems that I saw ISA + many more times then ISA -.

Ok, let's talk about the three ways that you can fly an airliner in cruise flight. These are "High Speed Cruise", "Constant Mach" and "Long Range Cruise".

"High Speed Cruise" is the speed that I used for most of my corporate flying. The speed was limited to VMO (max operating speed) or N1’s or Engine Temperature. These are "limitations" which told me how fast I could go on a particular day. You might say that Corporate Pilots have the "need for speed", as the passenger's time is more valuable than any fuel savings.

An efficient airline could not operate their fleet at "High Speed Cruise". They usually fly at "Constant Mach" or, in some cases, "Long Range Cruise".

"Constant Mach" is most important for using the "Nat Tracks". The idea is to maintain constant Mach speeds so those airliners can be placed in a line crossing the Atlantic. If everyone maintains the same Mach speed, there should be no problems. It is possible, however, to see a change in ISA in crossing the Atlantic. In this case you will have to adjust your power setting to maintain Constant Mach. Most airlines flying in the states use "Constant Mach". The controlling factor in flying this way is "Mach number".

"Long Range Cruise" is used when you have to get the most performance out of your airliner as regards to fuel burn per nautical mile. The controlling factor in flying this way is "fuel flows". When fly you at "Long Range Cruise" speeds it takes longer to arrive at your destination – but not as long as a fuel stop en-route

OK, I know you have been waiting for "Cap'n Ed's" ten-question quiz. :< )

Here goes!

1. ISA stands for?

a. Initial Speed brake Altitude.
b. Initialization and Stabilization Altitude.
c. International Standard Atmosphere.
d. None of the above.

2. If you are crossing the Atlantic using the "Nat Tracks" and fuel is NOT a problem you should fly your airliner at?

a. High Speed Cruise.
b. Constant Mach.
c. Long Range Cruise.
d. Any of the above.

3. Using my "Rule of Thumb" what is ISA for FL330?

a. -51 degrees C.
b. -81 degrees C.
c. -31 degrees C.
d. None of the Above

4. ISA is important for?

a."Step Climbing" your Airliner.
b. Determining what Mach to expect at altitude.
c. Determining what Fuel Burn to expect if we are flying at Constant Mach.
d. All of the Above.

5. ISA Plus Temperatures will give us?

a. Increased Performance.
b. Decreased Performance.
c. The Same Performance.
d. None of the Above.

6. "Long Range Cruise" is dependent upon?

a. Constant Mach.
b. The Limitations Section of the Flight Manual.
c. Fuel Flows.
d. None of the Above.

7. To figure out ISA using our "Rule of Thumb" you should?

a. Double the Altitude and add 15.
b. Double the Altitude and subtract 15.
c. Double the Altitude and add 30.
d. Double the Altitude and subtract 30.

8. You are flying the "Nat Tracks" to Europe and find that your Mach begins to "roll back". Everything is mechanically sound. Based on this, what is the likely occurrence?

a. You have entered air that is ISA minus.
b. You have entered air that is ISA plus.
c. You have entered the "Bermuda Triangle".
d. None of the above.

9. Based on question 8 what is the best solution?

a. Nothing, airliners behind will slow up for me.
b. Apply more power to maintain your assigned Mach.
c. Immediately descend to a lower altitude.
d. Any of the above.

10. If you are flying at "High Speed Cruise" what could be some of the limitations?

a. VMO / MMO (Max Certified Speed).
b. Max allowable N1s or N2s.
c. Max EPRs (Exhaust Pressure Ratios), or Max EGTs (Exhaust Gas Temperatures).
d. All of the Above.



OK, if you missed any correct answers then you have to take the test over again

:< (

Still Just Kidding

Best regards,

Ed Ward, Jr.
Executive Vice President,
Chief Pilot,
Head Of Training
WestWind, VA

Thanks to Mark Chapel for Proof Reading.


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