Airwothiness Notices - B045, Edition 1 - 15 May 1998

Compatibility of Multiple Modifications


The purpose of this notice is to remind aircraft owners, operators and maintainers of the requirements to ensure compatibility of the latest modification being installed with other modifications previously installed on the given aircraft.

Note: Although this issue predominantly applies to modifications, it could also apply to repairs in specific cases.

Regulatory Requirements

Federal Aviation Administration (FAA) and Transport Canada supplemental type certificates normally include a compatibility statement which states in part:

"Conditions:  Prior to incorporating this modification, the installer shall establish that the interrelationship between this change and any other modifications(s) incorporated will not adversely affect the Airworthiness of the modified product."

The compatibility requirements specified above equally apply to a modification for which its installation was approved under an FAA 337 Field Approval, or another form of approval acceptable to the Minister.


The following, although not an all inclusive list, provides examples of types of modifications or a combination thereof which could introduce some incompatibility, e.g. the installation of:

  1. Autopilots on aircraft fitted with lift devices or other changes which affect the flight characteristics of the aircraft, i.e. a combination of modifications which individually increase the drag of the aircraft.
  2. Satellite communications equipment and global positioning system (GPS) navigation equipment, i.e. avionics equipment which may have adverse interaction.
  3. Engine, propeller and airframe modifications which have not been specifically approved together.
  4. Several accessories mounted on the skid tubes of helicopters.
  5. Modifications which introduce new operating limitations or procedures that are contrary to those limitations or procedures specified in other flight manual supplements.
  6. Modifications which include several structural design changes and which may cause superposition of stresses, reduction in stiffness or changes to fail safe or damage tolerance characteristics.

Installer's Responsibilities

Persons involved with the installation of a latest modification should assess its functional and operational compatibility with other modifications already installed, and ensure that it will not introduce any adverse characteristics, such as physical interference or impact on the operation of other modifications installed.  Where the assessment required to ensure compatibility between all modifications installed exceeds the installer's capability, he/she should then enlist necessary engineering support to make that determination.  Where adverse interrelationship is evident, negotiations should be undertaken with the approval holder or a qualified person, e.g. a delegate, to resolve the unsatisfactory situation.

Where operating limitations inherent to specific modifications are conflicting, the most restrictive limitations shall be applied.  In some cases, the development of a new flight manual supplement may be required to prescribe the operating envelope applicable to the multi-modification configured aircraft.

A compatibility assessment of installed modifications should also be repeated whenever an aircraft is converted for operation in a different role or when a change in its basic configuration has occurred, e.g. changing from:  1)  cargo to a passenger configuration, 2)  landplane to seaplane configuration, 3) normal utility to restricted category, etc.

Statement of Compatibility

In addition to meeting the objectives set forth in a Conformity Certificate - Repair or Modification, 24-0045 form, the inclusion of a statement of compatibility by the installer would provide interested parties with assurance that compatibility of the latest modification with other modifications previously installed has been assessed and proven satisfactory.

For Minister of Transport

K.J. Mansfield
Director, Aircraft Certification