Weight And Balance Flashcards
What are index units?
- Used because moments can be very large numbers
- Divided by 10,000
- 1 index unit (1IU) = 10,000kg-mm
What is payload?
- AKA Commercial Load
- The load that the aircraft is paid to carry
Includes:
- Passengers
- Baggage
- Freight
What is specific gravity?
- AKA relative density
- Ratio of the density of a substance to that of a given reference material
What is specific gravity and weight of AVGAS and JET A-1
AVGAS:
Specific gravity = 0.72
1L of AVGAS = 0.72kg
JET A-1 (AVTUR):
Specific gravity = 0.77 to 0.83
1L of Jet A-1 = 0.80kg
What is the datum?
- Reference point from which all measurements of arms are taken.
- Can be located at any point and is determined by the aircraft manufacturer
What is an arm/moment?
Arm = moment arm
- Horizontal distance from the datum to the Centre of Gravity of each item
- The arm is positive if measured aft of the datum
- The arm is negative if measured forward of the datum
- Arm can also be referred to as a Station (STA)
What is a moment?
- Moment and ‘moment arm’ are not the same
- The moment is a turning force around a point
- The product of the weight of an item multiplied by its arm
Moment = Weight (force) x Arm (distance)
Centre of Gravity
- The point where all the aircraft’s weight is acting through
- Can be thought of as the pivot (balance) point of an aircraft
- Must lie within certain limits for an aircraft to remain controllable
Longitudinal CoG range and limits
- With a change in weight, there will be a corresponding change in the CoG position of the aircraft
- Will also change if the weight is moved
Centre of gravity envelope
- Shows the aircraft weight limits
Basic Empty Weight (BEW)
Include:
- Airframe
- Engine
- Fixed equipment used for all operations
- Unusable fuel
- Fuel oil and systems fluid (hydraulic fluid)
Empty weight CoG Position
- The CoG position of the aircraft at basic empty weight
Basic Operating Weight (BOW)
- AKA ‘Aircraft Prepared for Service Weight (APSW)
Includes:
- Basic Empty Weight (BEW)
- Pilot and crew
- Crew’s baggage
- Rations
Basic Operating Weight = Total weight of the aircraft - payload and usable fuel
Zero Fuel Weight
- Basic operating weight plus payload
Zero fuel weight = basic operating weight + payload
Therefore:
Zero fuel weight = total weight of the aircraft - usable fuel
Maximum zero fuel weight (MZFW)
- This is a limit
- Above MZFW, the aircraft needs the weight of furl in the wing tanks to prevent the wings from being overstressed while it is flying
- The weight of the fuel in the wings holds down the wings, opposing life which they produce. If nothing acts to counter the lift on the wings, the wings are in danger of separating themselves from the aircraft in flight
Ramp weight
Ramp weight = zero fuel weight + usable fuel
- Maximum gross weight allowed prior to taxiing.
Gross Weight
- AKA All Up Weight (AUW)
- Total weight of the aircraft at any time
Includes:
- Basic empty weight
- Pilot, crew and their belongings
- Payload and fuel
Take-off weight
- Gross weight at take-off
- Must be under Maximum Certified Take-off weight or the performance-limited take-off weight
Maximum Certified Take Off Weight (MCTOW)
- Maximum weight permitted at the beginning of the take-off run.
- Limitation
-Must not be exceeded under any circumstance
Maximum Certified Landing Weight (MCLW)
- Maximum weight permitted for landing
- Structural limitation which must not be exceeded
- MCLW is the same as the MCTOW in light aircraft
- MCLW may be less than the MCTOW in larger aircraft
Landing Weight
- Gross weight at landing
- Must not exceed maximum certified landing weight
Explain aircraft balance
- Lift acts through the Centre of Pressure, while weight acts through the Centre of Gravity
- Light-Weight couples wants to make the nose of the aircraft pitch down
- Thrust-Drag couple wants to make the nose of the aircraft pitch up
- Both the CoP and the CoG move in flight, the tail plane is there to balance any residual longitudinal pitching moments
Lateral fuel imbalance
- Occurs when one wing has more fuel than the other causing it to be heavier
- Pilot continuously needs to use aileron - causes more drag
- Causes aircraft to fly inefficiently
How does an aircraft prevent fuel imbalance?
- Cross feed system
- Lets fuel flow from one tank to the other over time
- Change tanks at nominated intervals
Swept wing aircraft
- Where fuel is stored in the wings
- As fuel is used from an outboard tank, the CoG shifts forward as the tank is behind the CoG
- Same consideration applies to an inboard tank where the fuel is in front of the CoG
- CoG will move rearwards, as the tank is in front of the CoG
Forward centre of gravity
- Long moment arm between CoG and the tail plane.
- Aircraft nose is heavy and stable longitudinally
- Too much forward CoG can reduce the manoeuvrability
Cog outside the forward limit
- Nose heavy - the nose of the aircraft will want to pitch down. Requires a lot of aft control column pressure.
- Unable to rotate or flare - there will not be enough elevator nose-up force to rotate the aircraft on take-off and flare on landing
- Harder to hold an attitude - the aircraft will not have the elevator nose-up force to hold an attitude
Aft centre of gravity
- Short moment arm between CoG and the tail plane
- Aircraft will be tail heavy and less stable longitudinally
CoG outside the aft limit
- Tail heavy - nose will want to pitch up. Requires forward control column pressure.
- Too light stick forces - the further back the CoG, the harder it will be to control the pitch of the aircraft.
- No forward elevator authority - not enough elevator nose-down force to counter the pitch up moment, particularly at low airspeeds. Can lead to a stall on take-off and landing which the pilot is unable to recover from
Heavy vs. Light aircraft
Heavy aircraft has:
- Higher take-off speed
- Higher stalling speed
- Poorer climb performance
- Lower ceiling
- Less manoeuvrability
- Higher fuel consumption
- Reduced cruise speed for a given power setting
- Higher landing speed
- Longer landing distance
- Greater braking requirements
US gallons conversions
1 US gallon = 3.785 litres
1 US gallon = 5.99 lb
1 US gallon = 2.72 kg
1 litre = 0.26 US gallons
1 litre = 1.58 lb
1 litre = 0.72 kg
Kilogram to pound conversion
1 kg = 2.205 lb
Calculating take-off weight
Add:
- Basic Operating Weight
- Fuel required for flight
- Payload
Calculating landing weight
Take-ooff weight - the fuel less burn off (fuel used for flight)
Three main limitations for weight and balance
- Maximum Certificated Take-Off Weight (MCTOW)
- Maximum Certificated Landing Weight (MCLW)
- Maximum Zero Fuel Weight (MZFW)
Maximum payload calculation
- MZFW - APS weight = Maximum payload available
- MCTOW - APS weight - Total fuel weight = Maximum payload available
- MCLW - APS weight - Landing fuel weight = Maximum payload available
Reasons for a change to payload
- Loading extra weight
- Offloading weight
- Load or offload weight to put the CoG at a given station
- Move weight from one station to another
- Load or offload weight at a specific station without exceeding CoG limits
Total weight x CoG position (“Moment Arm”) =
Total moment
Total weight + weight added =
New total weight
Weight added x STA (“Arm”) =
Moment
Total moment + moment =
New total moment
New total moment/new total weight =
New CG position
Total weigh - weight removed =
New total weight
Total moment - moment =
New total moment
New total moment/new total weight =
New CoG position
