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Private Pilot — Airplane
Private Pilot Oral Exam Questions: Weight & Balance and Performance
Every private oral includes a weight-and-balance problem and a performance scenario, usually built from your own cross-country plan: 'Add a 190-pound passenger and full fuel — are you within limits? How much runway do you need at this density altitude?' The arithmetic is the easy part. What separates a strong applicant is being able to explain what the numbers do to the airplane: how CG position changes stability, stall speed, and recoverability, and what heat, altitude, and humidity do to takeoff and climb.
Examiners also probe honesty with charts: did you use the actual conditions, interpolate properly, and apply a personal margin? 'Book numbers were flown by a test pilot in a new airplane' is a sentence worth saying out loud in your oral.
CG position: the 'why' behind the envelope
Forward CG makes the airplane more stable but less efficient: the tail must produce more downforce, which adds effective weight, raises stall speed, increases takeoff/landing distances slightly, and can leave you without enough elevator authority to flare at the extreme. Aft CG is the dangerous direction: the airplane becomes less stable in pitch, stall speed decreases slightly, cruise gets faster — but stall and spin recovery may become difficult or impossible beyond the aft limit. That asymmetry (forward = annoying, aft = potentially unrecoverable) is the correlation-level answer DPEs want.
Be fluent in the basic math: weight × arm = moment; CG = total moment ÷ total weight. Know how to do a weight-shift problem (weight moved × distance moved ÷ gross weight = CG change) because 'your passenger moves to the back seat in flight — what happens?' is a stock question.
Density altitude and performance charts
Density altitude is pressure altitude corrected for nonstandard temperature: the altitude the airplane thinks it's at. High, hot, and humid all raise it. Effects compound — the engine makes less power (less dense air to burn), the propeller produces less thrust, and the wings need more true airspeed for the same lift — so takeoff rolls stretch and climb rates sag dramatically. Standard sea-level conditions are 29.92 inHg and 15 °C; a 95 °F day at a 5,000-ft elevation airport can produce a density altitude over 8,000 ft.
On chart technique: use the worst realistic conditions (actual temperature, pressure altitude, weight, wind), interpolate rather than round in the optimistic direction, read every chart note (many assume a paved, level, dry runway and a lean-for-altitude mixture), and apply a personal factor — many instructors teach adding 50% to book takeoff and landing distances. Saying that unprompted demonstrates exactly the risk management the ACS asks examiners to evaluate.
Practice questions with answers & rationales
Q1. What happens to stall speed and stability as CG moves aft?
Answer: Stall speed decreases slightly (the tail carries less downforce, so the wing supports less effective weight), and longitudinal stability decreases — the airplane is less willing to return to trimmed pitch. Past the aft limit, stall/spin recovery may be impossible because the relieving nose-down pitching tendency is too weak. So aft CG is 'faster but more dangerous,' and the certification aft limit is partly a spin-recovery limit.
Q2. Why does forward CG increase stall speed?
Answer: With a forward CG, the horizontal stabilizer must generate more tail-down force to hold the nose up. That downforce acts like extra weight the wing must lift, so the wing reaches its critical angle of attack at a higher airspeed. Same logic explains slightly longer takeoff distances and lower cruise speed at forward CG.
Q3. Define density altitude, and tell me what today's is.
Answer: Pressure altitude corrected for nonstandard temperature — the air density the airplane actually experiences, expressed as an altitude. Compute it on your checkride day: set 29.92 to read pressure altitude (or apply about +/-1,000 ft per inch of Hg from 29.92), then correct roughly +120 ft per °C above standard for that altitude. Examiners expect you to produce a real number with the E6B or chart, not just the definition.
Q4. Your performance chart says 1,200 ft ground roll. The runway is 2,200 ft. Do you go?
Answer: The legal answer is yes; the right answer is 'let me think.' Book numbers came from a new airplane, a test pilot, and perfect technique. Apply a 50% margin (1,800 ft), consider obstacle distance (the 50-ft obstacle number may be much larger), runway surface and slope, wind, and your own proficiency. If the margins are thin, lighten the load, wait for cooler temperatures, or use a longer runway. The examiner is grading risk management here, not arithmetic.
Q5. You loaded the airplane at the forward limit. During flight your 170-lb passenger climbs from the front to the back seat, 36 inches aft. What happens to the CG?
Answer: CG shift = (weight moved × distance moved) ÷ gross weight. For example, in a 2,400-lb airplane: (170 × 36) ÷ 2,400 ≈ 2.6 inches aft. You'd verify the new CG is still inside the envelope. The follow-up the examiner wants: handling will change — slightly less stable in pitch, retrim required — and weight shifts in flight should have been planned on the ground.
Q6. Does burning fuel in flight change your CG? Does it matter?
Answer: Yes — as fuel burns, total weight drops and the CG moves along the fuel tank's arm direction (forward or aft depending on the airplane). You must verify the loading stays in the envelope for the entire flight: a takeoff-legal CG can drift out of limits by landing. Computing both takeoff and landing W&B for your cross-country shows the examiner you understand this.
Q7. How does humidity affect performance, and does your chart account for it?
Answer: Water vapor is less dense than dry air, so high humidity lowers air density, costing engine power and a little lift — most POH charts do not account for it. The honest answer: treat high humidity as extra density altitude and pad your margins. Knowing what your charts don't include is exactly the kind of judgment statement that ends a line of questioning well.
Common mistakes to avoid
- Arriving with a generic W&B instead of one computed for the actual checkride loading — you, the DPE's real weight, today's fuel. Many examiners weigh their flight bag just to see what you do.
- Being unable to explain why aft CG is dangerous — the spin-recovery answer, not just 'it's out of limits.'
- Using field elevation instead of density altitude in performance charts.
- Rounding chart values optimistically or ignoring chart notes (dry, paved, level runway; specific flap/mixture technique).
- Computing takeoff W&B only and never checking landing CG after fuel burn.
- Quoting book distances with no personal margin. Examiners are explicitly graded territory: the ACS risk-management elements expect you to add one.