RPL Exam: Weight & Balance and Performance Charts Explained

Weight and balance, and performance chart interpretation, together account for a significant portion of the RPL theory exam. They test whether you can calculate safe loading conditions and extract accurate performance figures before flight. Here is what CASA expects you to know.

Why weight and balance matters

Every aircraft has a maximum certificated weight and a defined centre of gravity (CG) range. Operating outside these limits affects stability, structural integrity, and performance.

• Stability: a CG too far forward increases stick forces and reduces pitch authority; too far aft makes the aircraft unstable and potentially unrecoverable
• Structural integrity: exceeding Maximum Take-off Weight (MTOW) increases stress on the airframe, wings, and landing gear
• Performance: a heavier aircraft has a longer takeoff roll, higher stall speed, reduced climb rate, and longer landing distance

Key weight terms

Before working through a loading calculation, you need to understand the standard weight terms used in the Pilot's Operating Handbook (POH) and on the CASA exam.

Term	Definition
Basic Empty Weight (BEW)	Aircraft weight as manufactured, including unusable fuel and full oil
Useful Load	MTOW − BEW (passengers, baggage, usable fuel)
Zero Fuel Weight (ZFW)	Loaded aircraft weight without usable fuel
Takeoff Weight (TOW)	Total weight at start of takeoff roll
Landing Weight	TOW minus fuel burned en route
Maximum Takeoff Weight (MTOW)	Maximum certified gross weight for takeoff

Centre of gravity calculations

CG is calculated using the moment method. The arm is the horizontal distance from the aircraft datum — a fixed reference point, often the firewall or front face of the propeller — to the item being loaded. The two core formulas are:

• Moment = Weight × Arm
• CG = Total Moment ÷ Total Weight

Step-by-step loading calculation

Follow these steps in order when completing a loading calculation in the exam or before flight:

1. List each item (pilot, passenger, baggage, fuel) with its weight and arm from the loading schedule
2. Calculate each item's moment (weight × arm)
3. Sum all weights to get total weight
4. Sum all moments to get total moment
5. Divide total moment by total weight to find CG location
6. Check: is total weight ≤ MTOW? Is CG within the forward and aft limits shown on the CG envelope?

Reading the CG envelope

The CG envelope is a graph showing the allowable CG range at different aircraft weights. The x-axis shows CG position (inches from datum or % MAC); the y-axis shows gross weight. Your calculated CG must fall inside the envelope at both takeoff and landing weight.

Effect of fuel burn on CG

As fuel burns during flight, both weight and CG change. Depending on the aircraft type, burning fuel can shift CG forward or aft. Always check that the landing CG position is also within limits — not just the takeoff position.

Exam trap: CG may be within limits at takeoff but outside limits at landing after fuel burn. You must verify both conditions.

Performance charts — what CASA tests

RPL candidates must be able to use the performance charts in the POH to extract accurate figures for planning purposes. This is frequently tested in the CASA exam. You will be expected to find:

• Takeoff ground roll and distance to clear a 50 ft obstacle
• Climb rate at various weights and density altitudes
• Cruise speed and fuel burn at a given power setting
• Landing distance

Density altitude and performance

Density altitude is the altitude the aircraft "thinks" it is at in terms of air density. Performance charts are based on pressure altitude plus a temperature correction. Understanding how density altitude affects performance is one of the most important concepts for the RPL exam.

• Hot day → lower air density → higher density altitude → longer takeoff roll, reduced climb rate
• High-altitude aerodrome combined with high temperature results in significantly degraded performance

Exam question type: Given aerodrome elevation 1,200 ft, QNH 1008 hPa, OAT 35°C — calculate density altitude, then read takeoff roll from the chart.

Calculating pressure altitude from QNH

Pressure altitude is the indicated altitude when the altimeter is set to 1013 hPa. When QNH differs from 1013 hPa, apply the following correction using approximately 30 ft per hPa:

• QNH 1008 hPa (below 1013): pressure altitude is higher than aerodrome elevation by approximately (1013 − 1008) × 30 ft = +150 ft
• QNH 1020 hPa (above 1013): pressure altitude is lower than aerodrome elevation by (1020 − 1013) × 30 ft = −210 ft

Takeoff distance charts

Most POH takeoff distance tables are based on a defined set of standard conditions. Always note the assumed conditions before applying the chart, then apply correction factors for your actual conditions.

Standard assumed conditions are:

• Standard atmosphere (ISA), sea level
• Maximum gross weight
• Paved, level, dry runway
• No wind

Apply correction factors for: gross weight (lower weight = shorter roll), headwind (reduces ground roll approximately 10% per 10 kt headwind), uphill slope (increases roll), and high temperature (increases roll).

Climb performance

Climb rate depends on excess thrust power — the difference between power available and power required. Three factors have the greatest effect on climb performance:

• Weight: a heavier aircraft has a higher stall speed, requires more lift, and has less surplus power available for climb
• Density altitude: the engine produces less power, the propeller is less efficient, and true airspeed is higher for a given indicated airspeed
• Configuration: flaps extended increase drag — a clean configuration gives best climb rate

V_Y vs V_X

Two climb speeds are commonly tested in the RPL exam. Understanding when to use each is important for both the exam and for safe departure planning.

• V_Y (best rate of climb): maximum altitude gain per unit of time. Used for en route climbs.
• V_X (best angle of climb): maximum altitude gain per unit of horizontal distance. Used to clear obstacles on departure.

Common exam traps

The following mistakes are frequently made by RPL candidates in weight and balance and performance questions. Review each one carefully before your exam.

• CG within limits at takeoff but outside limits at landing after fuel burn — always check both positions
• Density altitude can be higher than actual altitude on a hot day — performance is worse than the indicated altitude alone would suggest
• A headwind reduces takeoff roll, but a tailwind significantly increases it — a tailwind correction factor of approximately ×1.5 is commonly tested
• V_Y and V_X converge at the aircraft's absolute ceiling
• Weight units: always check whether the POH chart uses kg or lb, and whether arms are in inches or mm

Related Resources

• Weight & Balance Study Guide
• Performance Study Guide
• Performance Charts Explained