PAYLOAD CALCULATIONS FOR THE PILOT

Introduction

The PIC is responsible to ensure that none of the limitations laid down for an aircraft are ever exceeded during any stage of the flight - starting from ramp weight up unti landing weight. There are a large number of people doing the loading; preparing the load and trim sheet; and thus assisting the PIC with this job, but the final responsibility of ensuring that the aircraft is correctly loaded rests with the PIC. The PIC is given a load and trim sheet in which all details are mentioned. He must check that all loading is within the limitations for the operations that he is signing up to undertake, and are as per his/ her latest calculations. These limitations could be related to strength, performance or cg position. In this chapter we would only focus on the how to calculate the amount of payload that can be carried on a particular flight. To get a basic understanding, it is important to read through the following: -

• Strength limits are easy to understand and are given out in the Certificate of airworthiness/ Flight manual as Max. Ramp Weight, Max Take off Weight, Max Landing Weight and Max Zero Fuel Weight.
• Performance limitations are based on the environmental conditions in which the aircraft operates and changes at different places and also at the same place at different times of the day and seasons depending on changes in ambient temperature, pressure, runway, winds, precipitation, etc. These could be Climb Limited Weight (CLW), Obstruction Limited Weight (OLW) or Runway Length Limited Weight (RLLW) limitations.
• CG limits are decided by the manufacturer and are given out in the Certificate of airworthiness/ Flight manual. These should be respected in terms of forward limit and aft limit of CG, during the entire flight spectrum. CG going out of these limits could lead to stability and control problems for the aircraft. An out of limit aft CG can lead to stability problems and an out of limit forward CG to controllability problems. CG changes during the flight due to consumption of fuel, movement of passengers, extension of services, etc. At no stage should the CG be allowed to go outside the stated limits.

Relevant Weights (or Masses)

Basic Empty Weight: The measured or computed weight of an aircraft excluding the weight of all removable equipment and other items of disposable load but including engine coolant, fixed ballast and unusable & trapped fuel and oil. All aircraft weighing more than 2000 kgs are weighed at the time of issue of certificate or airworthiness and thereafter every 5 years as per the regulations (CAR Section 2 - Airworthiness

Series 'X' Part- II dated 14th May, 1993). In addition, if there has been any significant change in empty weight due to repair/ alterations, the aircraft is required to weighed.

Operational Empty Weight: BEW + Cabin Equipment + Crew & their baggage + Potable water and lavatory chemicals.

Dry Operating Weight: OEW + Catering; Newspapers, etc. The load and trim sheet, and thus the Captain starts working from this figure onwards. The rest of the information is to understand the process.

Zero Fuel Weight: DOW + Payload

Landing Weight: ZFW + Fuel Reserves (Alternate, holding and contingency fuel)

Take Off Weight: LW + Trip fuel

Ramp Weight: TOW + Taxy fuel

The table below gives the various figures in tabulated form and the weights are to be added from the bottom row upwards, flowing outwards from the middle column.

Traffic Load: Total mass of passengers, baggage and freight, also called as Payload, as revenue is generated from this load only in commercial aviation.

Useful Load: The total of traffic load and useable fuel.

Max Allowable TOW or Regulated TOW: The MTOW of an aircraft is fixed based on the structural strength. This is fixed by the regulator and does not vary with operating conditions. However, the aircraft cannot always take off at this maximum weight due to performance limitations. Consider the same aircraft operating from different pressure altitudes, say from Delhi and from Leh, or operating from Delhi in summers at 40°C or in winter at 10°C. This is why we have something known as RTOW, that varies with temperature, flap setting, altitude, length of runway and other environmental factors. To ensure that none of the limitations of the aircraft are ever exceeded during flight, it is important to ensure that the take off is always regulated by selecting the lowest of the following weights: -

• MZFW + Take-off fuel (Trip fuel + Reserves)
• TOW; Lesser of the Performance limited TOW or Structural MTOW.
• LW + Trip Fuel; Lesser of the Structural MLW or Performance limited LW.

Steps to find Payload

• Find the three figures for TOW based on MZFW, TOW, and LW, as given above
• Take the least of the three as the RTOW for that flight.
• Subtract the DOW from this to get the Useful Load.
• Subtract the Fuel Carried from this to get Payload.