Performance and Limitations

• Standard empty weight (GAMA) — airframe, engines, and all permanently installed operating equipment with fixed locations, including fixed ballast, hydraulic fluid, unusable fuel, and full engine oil
• Basic empty weight (GAMA) — standard empty weight plus the optional and special equipment that has been installed. This is the number your POH's loading form starts with, and it is airplane-specific
• Useful load — maximum gross weight minus basic empty weight; it is what you have to spend on people, bags, and fuel
• Payload (GAMA) — the weight of occupants, cargo, and baggage — useful load minus the fuel you actually carry
• Maximum gross weight — the maximum authorized weight of the aircraft and its contents
• Reference datum — an imaginary vertical plane from which all horizontal distances are measured for balance purposes
• Arm — the horizontal distance from the datum to the item, in inches
• Moment — weight × arm; the item's turning tendency about the datum
• Center of gravity — the point at which the aircraft would balance if suspended; total moment ÷ total weight
• CG limits — the forward and aft extremes within which the aircraft must be operated at a given weight

(PHAK ch. 10.) Teach arm as a lever and moment as leverage; students who understand that never confuse the two again.

• Computation method — build a table of weight, arm, and moment for each station; total the weights and moments; divide total moment by total weight to get CG. The method that always works.
• Table method — the POH gives moment (usually moment/1000) directly for common weights at each station; add the moments and enter the envelope chart.
• Graph method — the POH provides loading graphs from which moment is read for each station's weight, then the total is plotted on a CG envelope.
• Shift, add, and remove computations — for changes to a known loading.

Teach the computation method first even if the POH provides a graph. A student who learns only to read a graph cannot detect when the graph is being read wrong.

Forward of the forward limit: higher stall speed, higher control forces, greater elevator force needed to flare, higher fuel consumption from the increased tail-down force and resulting drag; in the extreme, insufficient elevator authority to raise the nose in the flare or recover from a stall.

Aft of the aft limit: reduced longitudinal stability, lighter and more sensitive controls (over-control), degraded stall recovery, and — in the extreme — an inability to lower the nose to break the stall; without the ability to decrease AOA, the aircraft continues in a stalled condition until it contacts the ground (PHAK ch. 5).

The same principle scales to the twin: VMC increases as CG moves aft, because the rudder's moment arm and therefore its effectiveness are reduced (AFH ch. 13).

Because fuel burns off from a station with its own arm. If the fuel arm is aft of the CG, burning fuel moves the CG forward; if forward, the CG moves aft. AI.II.F.S2 requires you to determine that W&B remains within limits during all phases of flight — so compute takeoff and landing conditions, and for a long flight, the worst-case point in between.

The classic trap: a load that is legal at takeoff and out of limits at landing, or vice versa. Teach the student to plot both points on the envelope, not just one.

• Atmospheric conditions — pressure altitude, temperature, and humidity, combined as density altitude; high, hot, and humid all reduce performance
• Pilot technique — chart values assume the exact speeds and configuration in the chart notes, flown by a test pilot
• Airplane configuration — flap setting, gear position, cowl flaps, propeller setting
• Airport environment — runway length, surface, slope, contamination, and obstacles
• Loading and weight and balance — weight affects nearly every number on the chart, and CG affects stall speed and control forces

Teach the direction of each effect before the arithmetic, so the student can sanity-check any answer they compute.

Ideal flight-test conditions explain the gap: chart numbers come from a new airplane flown by a test pilot, and it is unlikely that performance is duplicated in service (AFH ch. 13 makes this point directly about multiengine charts; it applies to all of them).

Teach a personal correction factor: add a margin to takeoff and landing distances as a matter of policy, and treat charted numbers as a floor on the runway you need, not a target. This is the same discipline AFH ch. 13 recommends for accelerate-stop distance — advisory data unless it appears in the limitations section, but experienced pilots insist on it as a matter of safety and good operating practice.

Data published in the limitations section of the AFM/POH is binding — 91.9 prohibits operating an aircraft without complying with the operating limitations in the approved flight manual. Performance data published elsewhere in the handbook is advisory unless the limitations section adopts it.

Practical example: most AFM/POHs publish accelerate-stop distances only as advisory; the regulations do not require the runway length to be equal to or greater than accelerate-stop distance, and it becomes a limitation only when published in the limitations section (AFH ch. 13).

Maneuvering imposes structural loads that a performance chart never shows, and that is the connection: aircraft are certificated to limit load factors by category — normal +3.8 to −1.52, utility +4.4 to −1.76, acrobatic +6.0 to −3.00 — with a 50 percent safety factor added, since the structure must support one and one-half times the limit load factor without failure (PHAK ch. 5). For aircraft over 4,000 pounds gross weight, the normal category limit is reduced.

And the operational consequence: stalling speed increases with the square root of the load factor, so a coordinated level 60° bank at 2 Gs raises stall speed about 41 percent (PHAK ch. 5).

Anchor it in a decision. Compute the takeoff distance for a standard day at your home field, then recompute for a summer afternoon at gross weight. Put both numbers next to the actual runway length and let the student see the margin disappear.

Then extend it to the parts of the flight the student did not think of: climb rate over terrain, the go-around from a high-density-altitude landing, and true airspeed versus indicated. For multiengine students, the density altitude conversation continues into VMC versus VS — with normally aspirated engines VMC decreases with altitude while stall speed does not, so at high density altitude the airplane may stall before losing directional control (AFH ch. 13).

• Using the wrong chart or the wrong line. Pressure altitude versus density altitude, and the wrong weight curve. Require the student to circle the chart's assumptions in the notes before reading it.
• Ignoring the chart notes. Speeds, flap settings, runway surface, and wind corrections are in the fine print and change the answer materially.
• Interpolating badly, or refusing to interpolate. Teach interpolation once, formally, and require it — not "round to the conservative side" every time.
• Computing takeoff only. The landing condition and the fuel burn are where out-of-limits loadings hide.
• Confusing usable and total fuel. The gauges read to zero, the tanks do not.
• Accepting "it flew fine last time." The most dangerous sentence in a preflight briefing, and a straightforward hazardous attitude — invulnerability, whose antidote is "it could happen to me."

Change the scenario after they finish. "Now your passenger's bag went in the back instead of the front — what moves, and by how much?" "Now it's 95 °F — is the runway still long enough?" "Now we're landing at the 2,800-foot strip — what has to come out of the airplane?"

That is authentic, learner-centered assessment (AIH ch. 6): it evaluates the ability to apply, not to recall. A student who can only reproduce the worked example has learned the form, not the concept, and will not catch the day the numbers do not work.

As a decision made on the ground with a written trigger, before external pressure arrives. The output of a performance calculation is not a number — it is one of three answers: go, go with a change (less fuel, fewer bags, a different runway, a cooler hour), or no go.

Make the student state the change they would make before you ask for it, and make them state the condition that would reverse the decision. AI.II.F.R1 through R6 — chart use, airplane limitations, calculated versus actual performance, exceeding weight limits, operating outside CG, and shifting weight — are all covered when the student can defend a specific decision with specific numbers.