Flight planning, weight and balance

In short

Flight planning is the process of deciding, before every flight, the route, cruising level, fuel, alternates and payload that make the flight legal, safe and economical; weight and balance is the parallel exercise of ensuring the aircraft's actual weight and its centre of gravity (CG) lie inside the manufacturer's certified envelope. The two are inseparable: the fuel the plan demands is weight, and that weight fixes both the maximum payload and the CG at every stage of the flight. In India the rules come from the Bharatiya Vayuyan Adhiniyam, 2024 (in force from 1 January 2025, replacing the Aircraft Act, 1934) and the Aircraft Rules, 1937, with the operating detail in DGCA Civil Aviation Requirements (chiefly CAR Section 8 Series on operations) aligned to ICAO Annex 6 and Annex 2. B.Sc Aviation students are examined on fuel policy, load and trim sheets, CG arithmetic and the ICAO flight plan form; the same work is done daily by flight dispatchers (Flight Operations Officers), load controllers and pilots at IndiGo, Air India, Akasa, SpiceJet, Air India Express, Alliance Air and every ground handler (AISATS, Celebi, Bird, Air India Airport Services).

Explained

Weight definitions and the weight chain

• Manufacturer's Empty Weight (MEW): structure, powerplant, systems and fixed equipment as built, no fluids.
• Basic Empty Weight (BEW): MEW plus unusable fuel, full engine oil and hydraulic fluid - the weight on the weighing report.
• Dry Operating Weight (DOW): BEW plus operator's items - crew and baggage, catering, potable water, documents, safety equipment. Does not include traffic load or usable fuel.
• Zero Fuel Weight (ZFW): DOW + traffic load (passengers, baggage, cargo, mail). Maximum ZFW (MZFW) is a structural limit protecting wing-root bending: fuel in the wings relieves bending, so weight that is *not* fuel is capped.
• Take-off Weight (TOW): ZFW + take-off fuel (= ramp fuel − taxi fuel). Landing Weight (LW): TOW − trip fuel.
• Traffic load (payload): ZFW − DOW. Allowed traffic load = the lowest of the limits computed below.
• Structural limits: MTOW (maximum take-off weight), MLW (maximum landing weight), MRW/MTW (ramp or taxi weight), MZFW. Performance limits (runway, climb, obstacle, en-route, tyre speed, brake energy) may be lower on the day and then they govern.
• Regulated take-off weight (RTOW): the lowest of MTOW, performance-limited TOW, MLW + trip fuel, MZFW + take-off fuel.

Standard masses and the Indian practice

• Actual weighing is always acceptable; standard (declared average) passenger masses are used by scheduled operators as approved by DGCA, with separate figures for male, female, child and infant, and an allowance for hand baggage. DGCA periodically revises these from survey data, so the exact kilograms in force must be read from the current CAR/operator manual rather than memorised; a commonly taught illustrative set is adult male ≈ 75 kg, adult female ≈ 65 kg, child ≈ 35 kg, infant ≈ 10 kg, plus checked-baggage averages by sector type.
• Small aircraft (under about 5,700 kg, general aviation and flying-school types used in B.Sc Aviation training - Cessna 152/172, DA40, Tecnam) must use actual passenger and baggage weights.
• Aircraft must be re-weighed periodically and after major modification or repair; the operator keeps a weighing schedule and a current weight-and-balance manual. Fleet (standard) DOW may be used where individual aircraft weights are close enough to a mean, within DGCA-approved deviation limits.

Balance: arms, moments and the CG envelope

• Datum: an arbitrary reference plane chosen by the manufacturer (often ahead of the nose). Arm (balance arm): distance from datum to the item, positive aft. Moment = weight × arm. CG = total moment / total weight.
• Mean Aerodynamic Chord (MAC): the chord of an equivalent rectangular wing with the same moment characteristics. Transport-aircraft CG is quoted as % MAC: %MAC = (CG arm − LEMAC) / MAC × 100, where LEMAC is the arm of the leading edge of MAC.
• CG envelope: a graph of weight against CG (in % MAC or arm) showing forward and aft limits; separate envelopes exist for take-off, in-flight and landing, and sometimes for flaps/gear configurations.
• Forward CG: more stable but higher stick force, larger download on the tail, higher stall speed and longer take-off/landing distance, more drag and fuel burn.
• Aft CG: less longitudinally stable, lighter controls, lower trim drag and better cruise economy, but risk of inadequate stability and difficulty recovering from a stall; aft of limit can make rotation uncontrollable.
• Fuel burn moves the CG because tanks are not at the CG; some long-range aircraft use a trim tank in the horizontal stabiliser to hold an aft CG in cruise for fuel saving.
• Index system: to avoid large numbers, moments are converted to a dimensionless index: Index = Weight × (Arm − Reference Arm) / K + Constant, where K is a scaling constant and the constant keeps indices positive. Load-and-trim sheets add index units graphically.
• Load shifting: moving a mass changes CG by Δ CG = (mass moved × distance moved) / total weight. Adding/removing mass: Δ CG = mass added × (arm of mass − old CG) / new total weight.
• Last-minute change (LMC): a late passenger/cargo change is entered on the loadsheet with its weight and index effect; operators set an LMC limit above which the sheet must be recomputed.

Fuel planning

Required fuel for a flight (ICAO Annex 6 / DGCA CAR Section 8 structure):

1. Taxi fuel - APU, engine start and ground manoeuvring at the departure aerodrome.
2. Trip fuel - take-off, climb, cruise, descent, approach and landing at the destination.
3. Contingency fuel - for unforeseen factors; the standard value is 5% of trip fuel (reducible to 3% with an approved en-route alternate, or to a statistical figure under an approved fuel-monitoring programme), with a floor of 5 minutes' holding at 1,500 ft above destination.
4. Alternate (destination alternate) fuel - missed approach at destination, climb, cruise, descent and approach/landing at the alternate.
5. Final reserve fuel - 30 minutes' holding at 1,500 ft above the elevation of the alternate (or destination if no alternate) at ISA for turbine aircraft; 45 minutes for piston aircraft.
6. Additional fuel - where needed to satisfy isolated-aerodrome or engine-failure/depressurisation en-route scenarios.
7. Discretionary (extra) fuel - at the commander's discretion.
8. Block (ramp) fuel = items 1-7. Minimum take-off fuel = block − taxi.
9. Alternate requirements: a destination alternate is normally mandatory; it may be dispensed with when the destination is isolated, or when the destination has an instrument approach and the weather for a defined window around ETA is comfortably above minima and the flight is short - the precise relief conditions and weather margins are in the operator's DGCA-approved operations manual.
10. Tankering: carrying cheaper fuel from a station where ATF is cheaper, accepting extra burn to carry it; viable only when the price differential exceeds the cost of the carried weight. Indian ATF prices vary by state because of VAT, so intra-India tankering is routinely evaluated.
11. Decision-point / re-dispatch procedures reduce carried reserves on long flights by re-planning in the air.
12. Fuel monitoring: DGCA requires the operator to monitor consumption and keep performance factors (deterioration allowance) current per aircraft.

Route, level and performance planning

• Inputs: AIP India and its supplements, enroute charts, NOTAMs, METAR/TAF/SIGMET/SIGWX and wind-temperature charts, aircraft performance manual, MEL/CDL items, aerodrome and runway data, ATFM measures.
• Route choice: ATS routes and conditional routes, flex/free-route airspace, danger and restricted areas, overflight permissions and charges; minimum off-route altitude (MORA/MOCA/MEA) for terrain clearance.
• Optimum level: trade-off of lower TAS-specific fuel flow at altitude against higher climb fuel and wind; step climbs as the aircraft lightens. Optimum level rises as weight falls.
• Cost index (CI): CI = time-related cost per hour ÷ fuel cost per unit mass; CI 0 = maximum range (minimum fuel), high CI = minimum time. The FMS converts CI to an ECON speed.
• Point of no return (PNR): the furthest point from which the aircraft can return to the departure (or a nominated) aerodrome with reserves intact. Equi-time point (ETP) / critical point: the point from which flight time to two nominated aerodromes is equal - driven by wind, used for diversion planning and ETOPS/EDTO.
• Performance limits for take-off: TORA/TODA/ASDA/LDA declared distances, V1/VR/V2, balanced field length, flap setting, bleed configuration, runway slope and surface condition, wind components, aerodrome elevation, temperature and QNH; flex/derate thrust reduces engine wear when the runway allows.
• Obstacle and climb limits: net take-off flight path with one engine inoperative must clear obstacles; WAT limit (weight-altitude-temperature) protects the certified climb gradients. Landing distance required includes the regulatory factor over available LDA and a wet/contaminated correction.
• En-route limits: one-engine-inoperative drift-down over terrain, oxygen and depressurisation planning.
• ETOPS/EDTO: twin-engine operations beyond a threshold distance from an adequate aerodrome need approval, en-route alternates with suitable weather, and extra critical-fuel scenario fuel - relevant to Indian wide-body and Gulf/Southeast Asia narrow-body operations.

Documents: the flight plan, OFP and loadsheet

• ATS flight plan (ICAO Form, "FPL"): filed with ATC; key fields are 7 aircraft identification, 8 flight rules and type of flight, 9 number/type of aircraft and wake turbulence category (L/M/H/J), 10 equipment and surveillance capability, 13 departure aerodrome and time, 15 cruising speed, level and route, 16 destination, total EET and alternates, 18 other information (DOF, PBN, REG, SEL, RMK, etc.), 19 supplementary information (endurance, persons on board, emergency equipment). Filed normally at least 60 minutes before EOBT in India; repetitive flight plans (RPL) are used for scheduled services.
• Operational Flight Plan (OFP) / computerised flight plan: the operator's document with waypoint-by-waypoint distances, tracks, winds, levels, planned fuel remaining, times and alternate plans - signed by the commander and the FOO.
• Load and trim sheet (loadsheet): states DOW, traffic load by compartment and zone, ZFW, fuel, TOW, LW, all against their maxima, plus the resulting ZFW/TOW/LW CG in % MAC and the stabiliser trim setting for take-off. Signed by the load controller and accepted by the commander; electronic loadsheets are the norm at Indian carriers.
• Loading instruction/report (LIR): tells the ramp where each unit load device (ULD)/bulk load goes and records what was actually loaded; deviations must be fed back before departure.
• Other dispatch paperwork: NOTAM and weather folder, MEL status, general declaration, passenger and cargo manifests, dangerous goods NOTOC, journey log book.
• Dispatcher licensing: DGCA issues the Flight Operations Officer / Flight Dispatcher licence (examinations, experience and training per CAR Section 7); joint pilot-dispatcher responsibility for the flight release is the Indian norm for scheduled operations.

Structural and operational consequences of overload and mis-trim

• Overweight: longer take-off run, reduced climb gradient and ceiling, higher stall and approach speeds, longer landing distance, higher tyre and brake loads, reduced manoeuvre margin, increased fatigue damage.
• Overload beyond MZFW stresses the wing root; beyond MLW it overstresses the landing gear at the certified descent rate.
• Mis-trim out of the envelope: nose-heavy aircraft may not rotate at VR or may need excessive elevator at landing flare; tail-heavy aircraft may pitch up uncontrollably at rotation - several accidents worldwide trace to incorrect load distribution or wrong ZFW entered in the FMS.
• Floor loading limits (kg/m²) and running load (kg/m) also constrain cargo even when total weight and CG are fine; cargo must be restrained for 9 g forward.

Key points

• DOW - empty weight plus crew and operator items; excludes payload and usable fuel.
• ZFW - DOW + traffic load; capped by MZFW to protect wing-root bending.
• TOW - ZFW + take-off fuel; LW - TOW − trip fuel.
• Allowed traffic load - the lowest of MTOW-, MLW-, MZFW- and performance-derived limits minus DOW and the relevant fuel.
• Moment - weight × arm; CG - total moment ÷ total weight; arms are measured from the datum, positive aft.
• % MAC - (CG arm − LEMAC) ÷ MAC × 100; the standard way to quote transport CG.
• Forward CG - stable, higher stall speed, more trim drag; aft CG - economical, less stable.
• Index - scaled moment used on load-and-trim sheets to keep numbers small and additive.
• Load shift - Δ CG = mass moved × distance moved ÷ total weight.
• Contingency fuel - normally 5% of trip fuel, 3% with an approved en-route alternate, minimum 5 minutes' holding.
• Final reserve - 30 minutes' holding at 1,500 ft for turbine aircraft, 45 minutes for piston.
• Block fuel - taxi + trip + contingency + alternate + final reserve + additional + discretionary.
• Cost index - time cost per hour ÷ fuel cost per kg; 0 gives minimum fuel, high gives minimum time.
• PNR and ETP - furthest point allowing return, and the point of equal flying time to two aerodromes.
• RTOW - the lowest of all structural and performance take-off weight limits for the day.
• WAT limit - weight restricted by altitude and temperature to meet certified climb gradients.
• ICAO FPL Field 15 - cruising speed, requested level and route; Field 18 - DOF, PBN and other data; Field 19 - endurance, POB and emergency equipment.
• Indian framework - Bharatiya Vayuyan Adhiniyam 2024 (replaced the Aircraft Act 1934) and Aircraft Rules 1937, DGCA CARs (Section 8 operations, Section 7 licensing), AIP India, aligned with ICAO Annexes 2 and 6.

Formulas and examples

Example 1 - Light aircraft CG (actual weights, Cessna-type trainer). Datum at the firewall; BEW 745 kg at arm 0.95 m; pilot + instructor 150 kg at 1.80 m; fuel 90 kg at 1.95 m; baggage 20 kg at 2.85 m. CG limits 0.95-1.35 m at this weight.

CG = 1,210.25 / 1,005 = 1.2042 m → inside 0.95-1.35 m, so the loading is acceptable. Now burn 70 kg of fuel: new weight = 935 kg, new moment = 1,210.25 − (70 × 1.95) = 1,210.25 − 136.50 = 1,073.75 → CG = 1,073.75 / 935 = 1.1484 m. Burning fuel aft of the CG moved the CG forward, still in limits.

Example 2 - Load shift to bring CG into limits. A 1,200 kg aircraft has CG at 2.55 m; the aft limit is 2.50 m. Two bags totalling 40 kg are moved from the rear locker (arm 3.60 m) to the forward locker (arm 1.20 m), a shift of 2.40 m forward. Δ CG = (40 × 2.40) / 1,200 = 96 / 1,200 = 0.08 m forward → new CG = 2.55 − 0.08 = 2.47 m, now inside the aft limit with 0.03 m margin. Check by first principles: original moment = 1,200 × 2.55 = 3,060 kg·m; change = 40 × (1.20 − 3.60) = −96 kg·m → 2,964 / 1,200 = 2.47 m. ✔

Example 3 - Allowed traffic load and payload limitation (narrow-body jet, illustrative figures). DOW 43,000 kg; MZFW 62,500 kg; MTOW 79,000 kg; MLW 67,400 kg; block fuel 9,500 kg of which taxi 200 kg and trip 7,500 kg.

• Take-off fuel = 9,500 − 200 = 9,300 kg.
• ZFW limit on payload = 62,500 − 43,000 = 19,500 kg.
• TOW limit: allowable ZFW = 79,000 − 9,300 = 69,700 kg → payload = 69,700 − 43,000 = 26,700 kg.
• LW limit: allowable TOW = 67,400 + 7,500 = 74,900 kg → ZFW = 74,900 − 9,300 = 65,600 kg → payload = 22,600 kg.
• Allowed traffic load = 19,500 kg (MZFW-limited).

Resulting weights: ZFW = 62,500 kg; TOW = 71,800 kg (under MTOW by 7,200 kg); LW = 64,300 kg (under MLW by 3,100 kg). If 180 passengers and bags average 100 kg each = 18,000 kg, spare capacity for cargo = 19,500 − 18,000 = 1,500 kg.

Example 4 - % MAC conversion and stab trim decision. MAC = 4.19 m, LEMAC at arm 17.00 m. Total weight 71,800 kg with total moment 1,268,000 kg·m.

• CG arm = 1,268,000 / 71,800 = 17.6602 m.
• % MAC = (17.6602 − 17.00) / 4.19 × 100 = 0.6602 / 4.19 × 100 = 15.76 % MAC.

If the take-off envelope at this weight is 12-32 % MAC, the loading is legal but near the forward limit: the trim table will call for a relatively high nose-up stabiliser setting, and the forward CG costs a little take-off distance and cruise fuel. Moving 600 kg of cargo from the forward hold (arm 12.0 m) to the aft hold (arm 24.0 m) gives Δ CG = 600 × 12.0 / 71,800 = 0.1003 m aft → CG 17.7605 m = 18.15 % MAC, a more economical trim.

Example 5 - Fuel plan and PNR. Trip fuel 7,500 kg for 2 h 50 min; taxi 200 kg; alternate fuel 1,100 kg; final reserve (30 min holding) 1,050 kg; no additional or extra fuel; an approved en-route alternate permits 3% contingency.

• Contingency = 3% × 7,500 = 225 kg; check the 5-minute floor ≈ 1,050 × (5/30) = 175 kg → 225 kg governs.
• Block fuel = 200 + 7,500 + 225 + 1,100 + 1,050 = 10,075 kg; minimum take-off fuel = 9,875 kg.
• With 5% contingency instead: 375 kg → block = 10,225 kg, i.e. 150 kg more carried and roughly 15 kg extra burnt just to carry it (rule of thumb ~3-4% of the extra weight per flight hour).
• PNR: safe endurance after reserves 4.0 h, outbound GS 420 kt (tailwind), homebound GS 340 kt.

D = 4.0 × 420 × 340 / (420 + 340) = 4.0 × 142,800 / 760 = 571,200 / 760 = 751.6 nm from departure; time to PNR = 751.6 / 420 = 1 h 47 min.

Where it is used

• Airline flight dispatch (OCC): FOOs at IndiGo, Air India, Air India Express, SpiceJet and Akasa prepare OFPs, fuel plans, NOTAM/weather folders and file ATS flight plans using systems such as Lido/mFlight, Jeppesen JetPlan or NAVBLUE.
• Load control and ramp: load controllers at AISATS, Celebi, Bird, Indo Thai and airline in-house teams issue loading instructions and electronic loadsheets, manage ULD distribution and LMCs within the departure slot.
• Flight crew duties: acceptance of the loadsheet, FMS entry of ZFW/TOW/CG, stabiliser trim setting, flex/derate thrust computation from RTOW tables or performance apps on EFBs.
• General aviation and flying training: DGCA-approved FTOs (IGRUA Raebareli, CAE Gondia, Redbird, Chimes, Madhya Pradesh Flying Club) require every student to compute weight, CG and fuel before solo flights in Cessna/DA40/Tecnam aircraft.
• Cargo and charter: Blue Dart Aviation, Quikjet and business-jet operators handle floor-loading limits, running loads, dangerous goods and tankering decisions where state VAT makes ATF cheaper.
• Airport and regulatory roles: AAI ATC flight-data units process FPLs and RPLs; DGCA flight operations inspectors audit fuel policy, loadsheet accuracy and standard-mass approvals; MRO and continuing-airworthiness staff maintain weighing records and update DOW after modifications.

Interview questions

• Q: What is the difference between Dry Operating Weight and Zero Fuel Weight? A: DOW is the empty aircraft plus crew and operator items but no payload or usable fuel; ZFW is DOW plus the traffic load, still excluding usable fuel.
• Q: Why does an MZFW limit exist at all? A: Fuel carried in the wings relieves wing-root bending in flight, so any weight that is not wing fuel must be capped to keep bending stresses within limits.
• Q: How do you convert a CG arm to % MAC? A: Subtract the LEMAC arm from the CG arm, divide by the MAC length and multiply by 100.
• Q: What are the components of block fuel? A: Taxi, trip, contingency, alternate, final reserve, any additional fuel for isolated-aerodrome or en-route scenarios, and the commander's discretionary fuel.
• Q: How much is contingency fuel and when can it be reduced? A: Normally 5% of trip fuel; it may be reduced to 3% with an approved en-route alternate, or to a statistical value under an approved fuel-consumption monitoring programme, subject to a 5-minute holding floor.
• Q: Why would an airline load an aft CG if it is less stable? A: An aft CG reduces the download the tailplane must produce, cutting trim drag and fuel burn - so operators aim for an aft-but-in-limits CG.
• Q: An aircraft is 300 kg over MLW but within MTOW. What are your options? A: Offload payload, carry less fuel, or plan a longer route or hold to burn the difference - the simplest planning fix is to reduce traffic load so that TOW minus trip fuel is within MLW.
• Q: What is a Last Minute Change and how is it handled? A: A late change to passengers, baggage or cargo after the loadsheet is issued; it is written on the loadsheet with its weight and index effect, within the operator's LMC weight and CG limits, otherwise a new loadsheet is produced.
• Q: What is cost index and what does CI = 0 mean? A: The ratio of time-related cost per hour to fuel cost per kg; CI = 0 tells the FMS to fly the maximum-range (minimum trip fuel) speed, ignoring time cost - note this is not the maximum-endurance speed.
• Q: Define point of no return and equi-time point. A: PNR is the furthest point along the route from which the aircraft can return to a nominated aerodrome with reserves intact; ETP is the point from which flight time to two nominated aerodromes is equal.
• Q: Which ICAO flight plan field carries endurance and persons on board? A: Field 19, the supplementary information, which is held by the filing office for search and rescue and not transmitted with the FPL.
• Q: Why does the CG move as fuel burns, and how is it handled on long-range aircraft? A: Tanks are not located at the CG, so burning fuel shifts it; some aircraft use a trim tank in the horizontal stabiliser to transfer fuel aft in cruise and keep an economical CG.
• Q: Which standard passenger masses would you use for a 9-seat charter? A: Actual weighed masses - standard average masses are only approved for larger aircraft with enough seats for the statistics to hold.

Common mistakes

• Including usable fuel or payload in DOW, or forgetting crew and catering so the DOW is too low and the loadsheet understates TOW.
• Adding fuel to ZFW to test the MZFW limit - MZFW is checked against ZFW only; fuel weight is compared against MTOW and MTW.
• Forgetting to deduct taxi fuel, so TOW is computed from ramp fuel instead of take-off fuel.
• Ignoring the MLW check: a flight legal at take-off becomes illegal at landing on a short sector where little trip fuel is burnt.
• Sign errors with arms - treating a forward arm as positive, or measuring from the nose when the datum is elsewhere.
• Mixing units: kg with lb, metres with inches, index units with kg·m; and quoting a CG in metres against a limit in % MAC.
• Computing contingency on block fuel rather than trip fuel, or omitting the 5-minute holding floor.
• Assuming the CG stays put through the flight, and skipping the in-flight and landing CG checks after fuel burn.
• Treating the cleared structural weight as the day's limit when runway length, temperature, slope or obstacles give a lower performance-limited weight.

Related topics

Air navigation: charts, dead reckoning and radio aids · Aviation meteorology · Aircraft performance and aerodynamics · Airport ground handling and ramp operations · Air traffic control and airspace rules · Aviation regulations: DGCA CARs and ICAO Annexes