L/D_MAX

But it isn't up to me and, at least in the United States, there are no regulatory requirements for any of this. If you have an angle of attack indicator, you are in pretty good shape. Even if you don't, you should be able to come up with a chart of your own to help you out should you ever need a maximum power off glide speed. By the way, if you don't have an AOA indicator, you really need to memorize a few critical deck angles should your pitot-static system ever go on strike. (For example, what pitch angle is required for level flight at takeoff power all engine and engine-out?)

First we'll explain the aero behind L/D_MAX and then a little about how to determine your own particular L/D_MAX AOA. If you don't have an AOA indicator, we'll cover how to build a chart.

Yes, engines are more reliable these days. Yes, I am paranoid. No, you don't have to do any of this. But you should.

1 — AOA, lift, drag

2 — L/D_MAX

3 — T-38 example

4 — Finding your aircraft's critical AOAs

Angle of Attack

The angle of attack is the angle between the chord line of the wing and the relative wind. More about this:

• Angle of Attack
• Everybody Has an Angle (of Attack)

Lift

A wing produces a force upon the aircraft known as "aerodynamic force." This force can be broken into two components: lift acts perpendicular to the relative wind, drag parallel to it. More about this:

• Lift
• Daniel Bernoulli

Drag

Part of the aerodynamic force generated by the wing points aft and parallel to the relative wind is known as induced drag. This type of drag starts high and decreases with airspeed. Another type of drag, known as parasite drag, is just what you think of when you think of air resistance against an object. Parasite drag starts low and becomes very high. More about this:

• Drag

The good folks at Northrop equipped the T-38 Talon with an Angle of Attack indicator with all the good stuff right on the gauge:

• Maximum Range: 0.18 AOA
• Maximum Endurance: 0.30 AOA
• Optimal Approach: 0.60 AOA
• Buffet Warning: 0.90 AOA
• Stall: 1.00 AOA

Chances are your business jet or airliner isn't so equipped. Too bad, this is the kind of stuff that can save your life. Not to worry, most aircraft seem to have the exact same numbers as that T-38 when it comes to AOA. You can easily find at least two of them.

Your manuals

Your aircraft manuals may have clues for some of your important angles of attack. In the case of the G450, this is all we have:

A small dose of aero

If you feel so inclined, you can devise a "V-G" diagram for your aircraft that plots velocity against G-loads, to determine your manuevering speed. See Operating Flight Strength for a primer on how to do this. Then look at Approach Speed Additives for the math that tells you an aircraft like a G450 with two engines and a 2.0 G loading limit on approach has an AOA of 0.66 at V_REF. Note: I used 0.67 AOA when rounding to the nearest number, but I've since been convinced rounding down would be more conservative.

I've watched AOA closely in every Gulfstream I've flown and 0.66 AOA at V_REF looks right, but since we usually flight with a wind additive, the AOA will be more like 0.50 on approach.

Other aircraft

Newer Gulfstreams have better discussions about aerodynamics and the following quote from the GVII-G500 Aircraft Operating Manual illustrated the 0.66 number's value: "For max endurance (e.g. holding or loitering), the aicraft should be flown at 0.66 AOA (allowing good maneuver and stall margin) at an altitude that has the best combination of low temperature (good) and low Mach number (also good)."

Putting it all together

You should record your aircraft's performance over several flights and try to collect multiple data points for identical configurations. I've done that for the G450 and here's what I found:

• Takeoff Rotation: 0.2 to 0.3
• Cruise (0.80 Mach): 0.15 to 0.20
• Cruise (LRC): 0.30
• Pattern: 0.40
• V_REF: 0.66
• V_REF "plus": 0.50

By definition all of these except 0.80 Mach are regardless of weight.

A common misconception is that since you stall at 1.00 AOA and your approach speed is 1.30 times stall speed (for most, but not all aircraft), then your approach angle of attack should be 1.00 / 1.30 = 0.77 AOA. But that is not the case, the 1.30 is based on speed not angles. Approach angle of attack tends to be between 0.50 and 0.60.

I know, from doing the math, that the AOA for V_REF is 0.66. But by the time you add at least 5 knots (because that is minimum approach speed), you end up with something less. If you want to see the math: Approach Speed Additives. (This is for a GVII, but the math is the same for the G450.)

I am often asked what AOA to fly in the event both engines flame out. We know this should be (L/D)_MAX but our AOA indicators don't show this. We also know that in some Gulfstreams, we are told 0.66 is the right number but that number includes maneuver and stall margins. My opinion: fly an AOA of 0.66 if you need to maneuver or winds could make a stall margin important. If you really have to stretch the glide, an AOA between 0.66 and 0.75 (where the Pitch Limit Indicator appears) might be the way to go.

For those without an AOA indicator

Your process is the same, but you will end up with airspeeds that will change with weight. There is talk at the FAA to make angle of attack indicators mandatory equipment. Let's hope that happens.

Air Training Command Manual 51-3, Aerodynamics for Pilots, 15 November 1963

Dole, Charles E., Flight Theory and Aerodynamics, 1981, John Wiley & Sons, Inc, New York, NY, 1981.

Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013.

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Gulfstream G450 Performance Handbook, GAC-AC-G450-OPS-0003, Revision 20, November 30, 2011

Technical Order 1T-38A-1, T-38A/B Flight Manual, USAF Series, 1 July 1978.

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