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Lesson 3 · Flight Performance & Planning

Climb Performance

Rate vs. angle, Vx & Vy, excess power and the ceiling — EASA PPL theory

⏱ ~20 min ✈ SEP · VFR 📋 EASA Part-FCL

1 — Two Ways to Measure a Climb

A climb can be described two completely different ways — mixing them up is the most common climb-performance mistake.

Rate vs. angle — the same climb, two questions
θ climb angle Height gained Ground distance RATE = ft per minute (what the VSI shows)

Rate of climb is height gained per minute (ft/min — what the VSI shows): good for “how long to my cruise altitude?”. Angle (gradient) is height gained per unit of ground distance (ft/NM or %): good for “will I clear that ridge?”. For a given rate, flying faster over the ground makes the path shallower — which is why obstacle climbs are flown slow.

2 — The Two Climb Speeds: Vx and Vy

Where each speed lives on the climb curves
Indicated airspeed → Climb performance → Vx Vy Angle of climb Rate of climb

Vx (best angle) gives the most height per ground distance — the steepest path, for clearing obstacles. It is the slower speed. Vy (best rate) gives the most height per minute — reaching altitude in the least time. It is the faster speed.

Memory hook
X = eXtra steep, for obstaXles (slower). On a typical SEP: Vx ≈ 60 kt, Vy ≈ 74 kt.

EASA exam trap
“Best angle” (Vx) is the slower speed — not the faster one. Best rate (Vy) is faster. Don’t let the wording fool you.

3 — What Actually Makes You Climb: Excess Power

An aeroplane climbs by converting spare engine power into height.

Rate of climb depends on excess POWERRate of climb ∝ (Power available − Power required) ÷ Weight
The power curves — the gap is your climb
Indicated airspeed → Power → Max excess power = best RATE (Vy) Vy Power available Power required

Why the ceiling exists
As you climb, the engine makes less power (thin air) while power required barely changes. The curves squeeze together; when they touch, excess power = 0 — that’s the absolute ceiling.

Power vs. thrust
Excess power sets the best rate (Vy); excess thrust sets the best angle (Vx). They peak at different speeds — which is why Vx and Vy differ.

4 — Density Altitude & the Ceiling

High density altitude robs the engine and wing, shrinking rate of climb and lowering the ceiling. Explore it:

Interactive climb explorer
Sea level16 000 ft
ISA −15 (cold)ISA +25 (hot)
80% (light)100% MTOW
Density altitude
0
ft
Rate of climb now
720
ft / min
Service ceiling
13 500
ft (ROC = 100 ft/min)

Service vs. absolute ceiling
Absolute ceiling = where ROC reaches 0 ft/min. Service ceiling = where ROC falls to 100 ft/min — the practical limit, and the figure quoted in the POH.

Converging speeds
As you climb, Vx (IAS) increases and Vy (IAS) decreases. At the absolute ceiling they meet — one airspeed maintains altitude, with zero excess to climb.

5 — Worked Example (Exam-Style)

Climbing through 4 000 ft PA, OAT +13 °C, POH ROC 500 ft/min, groundspeed 75 kt:

  1. ISA at 4 000 ft = 15 − 8 = 7 °C.
  2. ISA deviation = 13 − 7 = ISA +6.
  3. Rate → gradient: in one minute at 75 kt you travel 75 ÷ 60 = 1.25 NM and gain 500 ft.
  4. Gradient = 500 ÷ 1.25 = 400 ft/NM (≈ 6.6%).

The wind insight
Rate of climb (ft/min) does not change with wind, but gradient over the ground does: a headwind steepens the path, a tailwind flattens it. That’s why obstacle departures are flown into wind.

Knowledge Check

Question 1
Trees at the end of the runway after takeoff. Which speed clears them best?
Question 2
For a typical SEP near sea level, how do Vx and Vy compare?
Question 3
Rate of climb is directly proportional to:
Question 4
The service ceiling is the altitude where max rate of climb falls to:
Question 5
Steady 500 ft/min climb; a headwind increases. Rate and gradient?

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