No technical objections (NTO) are normally used in the context of the approval process of flight conditions for the likes of flight extensions and ferry flights.
An NTO is generally issued by the type certificate (TC) holder to support the ferry of an aircraft to a base where a certain defect can be rectified or to permit the extension of the due date for a scheduled task or activity. NTOs are not considered as ‘acceptable technical data’ by airworthiness authorities who will not issue changes, repairs, flight permits, waivers etc based on NTO letters.
For the airworthiness authorities a NTO letter issued by a TC holder assumes that the relevant engineering branches of an original equipment manufacturer (OEM) have made a risk assessment on a particular damage or a deferred maintenance item and have sent out an official opinion on criticality. NTOs that are without the signature of a responsible engineer behind the documentation and without a risk assessment from the OEM side are not accepted.
How NTOs are used
The procedure for granting an NTO requires that the aircraft operator requests this service to the OEM. The acceptance or denial options are exercised by the OEM after several factors are considered which are involved with granting the NTO, the most important of which are items that include airworthiness limitations, certificate maintenance requirements, or otherwise mandated by the applicable airworthiness requirements. Items may not be extended beyond normal scheduling tolerances. The OEM may only issue an NTO letter to confirm it has no objection for example to a flight extension. It is the operator’s task to obtain the actual flight extension from the competent authority.
The granting of an NTO letter by an OEM for a ferry flight requires that the aircraft operator requests this service to the OEM, who will consider the request on taking into account factors such as the distance requested to be flown to reach the maintenance facility, any inspection items which may have lapsed, the amount of time the aircraft has been inactive, where and how it has been stored, as well as other information necessary to determine the current technical state of the aircraft. Provided it can be issued, the NTO letter will be given to the operator outlying what (if any) inspections or other maintenance actions need to be accomplished prior to the ferry flight and what inspections can be deferred and the duration of deferment. This will allow the operator the flexibility to relocate the aircraft to a maintenance facility. It is the operator’s task to obtain the ferry flight permit from the competent authority.
Issues involved with NTOs
NTOs are generally used in the context of flight conditions’ approval; however, when there is a connection between two design organisations that are concurring on the same topic, the primary design organisation could issue an NTO to confirm that there is no safety impact on the existing design. In that case the NTO will support the compliance demonstration without constituting means of compliance.
Under European Union Aviation Safety Agency (EASA) airworthiness requirements, repair design data to permanent or temporary repairs under the provisions of Part-21 on ‘unrepaired damage’ must be approved. NTOs from the type certificate (TC) holder or any similar form of agreement with the TC Holder are not acceptable and do not constitute legal certification in accordance with Part-21 dealing with initial airworthiness or other applicable EASA decisions.
Similarly, under the airworthiness requirements of the relevant section of the U.S. Code of Federal Regulations (14 CFR), an NTO letter or statement that a particular maintenance or alteration action is design engineering representative (DER) approvable does not constitute Federal Aviation Administration (FAA) approval. However, such a statement when supported along with other technical information may provide the basis for a field inspector or delegate approval. When evaluating the compliance of data representing a particular maintenance or alteration action or continue-in-service condition, the data must include substantiated data to support the approval.