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Aircraft Load and Trim
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|Tag(s)|| Overrun on Take Off|
Overrun on Landing
It is crucial to the safety of an aircraft in flight that it is loaded in such a way that the specified maximum allowable weights are not exceeded and that the centre of gravity as loaded will be and remain within the permitted flight envelope for all stages of the intended flight. Once these conditions have been satisfied, it is equally crucial that the flight crew are aware of the prevailing weight and centre of gravity so that they can make appropriate settings to aircraft equipment; these include take off reference speeds, trailing edge flap, and pitch trim position. This is important to ensure that rotation can be made at the right indicated airspeed and will result in a successful transition from ground to flight meeting any restrictions imposed by the TORA and obstacle clearance, and with full control of the aircraft retained. It is also very important that aircraft baggage and freight load complies with the restrictions on carriage of dangerous goods.
It is essential that the Dispatcher or other official assigned responsibility for overseeing aircraft loading, specifies the loading requirement correctly and has a reliable method by which he/she can be satisfied that his/her instructions have been carried out as requested. Whilst modern methods are likely to use automated systems to determine the seating options for passengers and the disposition of dead load between available loads, effective procedures and compliance remain the only way of ensuring that what has been specified and passed to the aircraft commander has actually been achieved. For Hold Loading, this is usually achieved by the completion of a Loading Instruction Form (LIF). The LIF is given to the loading supervisor who certifies that it has been complied with and returns it to the issuer as evidence that the work has been completed. The completed load and trim sheet are then given to the aircraft commander. The human supervisor must also have a reliable means of confirming that the dangerous goods regulations and any special requirements for securing unusual items in the holds or in the passenger cabin have been complied with.
Load and Trim Sheets
The traditional requirement, which dates from the days when all load and trim sheets were completed manually on specific forms designed for use with each aircraft type, is as follows:
- the completed document is presented to the aircraft commander
- the aircraft commander checks that it is internally consistent by carrying out some simple cross checks of input and calculated data for gross errors and,
- if the cross checks are satisfactory, the commander formally accepts the load and trim sheet by means of a signature on at least two copies, one being retained by the departure agent and the other by the flight crew.
The process with DCS is slightly different in that only the input data need be checked and the completed document may not necessarily be signed by the agent presenting it, as he/she may have had no part in its preparation.
In both cases, however, the acceptance of an apparently correct load and trim sheet does not by its existence provide any assurance that the aircraft has necessarily been loaded as stated.
Departure Control Systems (DCS)
Most Load and Trim Sheets used today in commercial air transport flown by multi crew aircraft are produced by contracted Handling Agents by making flight-specific inputs to a proprietary DCS. There are a number of commercial DCS products available. Some are operated by large airlines for their own use and then also employed to generate external user business. Other similar DCS are operated independently of any particular airline. Where DCS systems are used, the data input and electronic generation of the load and trim sheet may be carried out at a regional centre and merely printed off - together with corresponding Loading Instructions - by the aircraft operator or the contracted handling agent employees.
Manual Load sheets
Manual Load sheets involve a pro forma calculation of Maximum Ramp Weight (MRW), Maximum Take Off Weight (MTOW) and Maximum Landing Weight (MLW) whilst the centre of gravity is located by marking the requisite aircraft operating weight (vertical scale) on a ‘drop line’ located on a centre of gravity ‘index’ scale which forms the horizontal axis. If the position so found is within the areas shown as the permitted safe flight envelope, then operation as loaded is possible.
Whilst manual preparation of load and trim sheets used to be the main method, they are now used so infrequently that recalling the necessary method for their completion can be challenging to ground staff. Many younger pilots have seldom or never prepared a manual load and trim sheet or checked one for acceptance; this unfamiliarity significantly increases the risk of undetected errors with significant consequences.
Aircraft Commanders' Acceptance of Load and Trim Sheets
The aircraft commander must be given a copy of the completed load and trim sheet for the flight and should check and sign it, leaving a copy at the point of departure. The aircraft commander is obliged to accept that the aircraft is loaded as stated in respect of the Hold Loading but in respect of Passenger Cabin Loading the senior cabin crew member usually confirms the number of passengers actually on board by means of a headcount after boarding has been completed.
Electronic Flight Bag generation of Load and Trim data
For some flights, especially but not only cargo flights, the flight crew have an electronic flight bag (EFB) which they use to calculate aircraft performance data, which takes account of the completed load and trim sheet. They also use the EFB to make the load and trim calculations themselves, so that once it has been checked, all that is required is that a copy be left with the agent at the point of departure. Clearly, it is vital that a rigourous process of cross checking is included in the preparation of such documentation.
Provisional and Final Load Sheets
The rise of DCS systems and the communication facility afforded by ACARS has allowed aircraft commanders to be given substantially complete and correct loading documents with ‘provisional’ status in plenty of time before STD; ‘final’ status documents with highlighted minor amendments can be generated as the aircraft leaves the gate for acceptance via ACARS at any time before take off commences.
Risks arising from aircraft loading
The primary risks arise from the aircraft being ‘set up’ for take off with pitch, trim and/or take off reference speeds which are not correct. This can arise in one of three ways:
- The aircraft is not loaded in the way stated on the accepted load and trim sheet (any load sheet type)
- The aircraft load and trim sheet uses correct input data but the output data is wrong (manual load sheets)
- The flight crew apply the (correct) load and trim data incorrectly when using it to calculate pitch trim, or reference speed data.
- The hold load is not properly secured or contains prohibited or incorrectly packed items.
Either actual misloading of an aircraft or incorrect use of correct load related data for aircraft systems set up can severely affect aircraft control. Loss of Control may occur during an attempted take off or during subsequent flight because either:
- an attempt (usually inadvertent) is being made to operate the aircraft outside the AFM limits, or
- the actions of the flight crew to control the aircraft are ineffective because the aircraft is not in the condition of load and or trim which is believed to prevail and/or has been used to set up key aircraft control parameters. These may be manually interpreted, e.g Vr on the ASI, or automatically computed from erroneous FMS inputs.
- European Action Plan for the Prevention of Runway Excursions Edition 1.0, January 2013.
For a more detailed discussion of the issues arising with hold and cabin loading, see Hold Loading and Passenger Cabin Loading which also have links to reports of investigations into specific outcomes consequent upon hold or cabin loading data being incorrectly calculated or applied.
Other articles of interest within SKYbrary include: