Aircraft in the Register are graded in 5 categories.

1. Benchmark (7 or more points)
These are the airframes which score the highest number of points by virtue of their significance, rarity, condition, current level of care and quality of conservation.
Although not necessarily the oldest, rarest and the most interesting of the breed, they will often be so. It may be appropriate to consider these airframes as the core of a composite ‘National Aeronautical Collection’.

2. Significant (5 or more points)
These are the airframes that go some way towards qualifying for the first category, perhaps by virtue of some special interest factor or the environmental conditions under which they are housed. Inclusion for an airframe in this group should be seen as positive encouragement for the organisation concerned to invest in its long term care and/or restoration.

3. Noteworthy (Up to 4 points)
These are lesser examples, but many would undoubtedly benefit from improved levels of protection and conservation. Reference to the world population figures may well explain an airframe’s inclusion in this category.

4. Of interest
Ungraded heritage assets such as privately owned aircraft whose owners have not applied to join the NAHR formally, and entries on the AHUK Register of ‘Anonymous’ Aircraft.

5. Ungraded
indicates that insufficient information is currently available to award a grade.

One point can be awarded to each airframe under the following nine headings, six of which are based on “Heritage Value”. Three of the headings, best described as “Collection Care”, may score double (marked *). This is to highlight and recognise improvements that are within the control of the museum collection, such as investment in workshop facilities and /or environmental conditions, or to acknowledge a special local or regional significance.

A - World Significance
The airframe represents a major step forward in the progress of aviation technology and/or history or has undertaken a flight/operations of world significance. Mention of this machine, or a close variant, could be expected in a general history of aviation published anywhere in the world.

B - National Significance
As above, but on a UK scale. Note that while the prototype Spitfire, had it survived, would be regarded as being of national (and indeed world) significance, that does not necessarily make the other 22,000 plus that were built of the same stature! For example, most variants of the Gloster Meteor are seen as being of significance, ie the day-fighters and the two-seat radar equipped night-fighters, but trainer and recce variants are not seen in the same vein, with the exception of the test-bed FR.9, for obvious reasons. It is important to remember that rarity on its own does not necessarily constitute worthiness.

C - Operational or Technical Significance
The airframe represents the development and application of new technologies, concepts, types of powerplant etc, constitutes an important variant within the type or has undertaken important flights, services, operations etc.

D - Social Significance
The airframe is associated with an important person, organisation or specific event. The inclusion of the heading inevitably demands some value judgements.
For example, Churchill’s Avro York would certainly qualify, but a rock singer’s private jet probably not.

E - Rarity
This relates to the rarity of the airframe, in national and/or world terms. It signals that it would be a very serious matter if such an airframe were to be ‘lost’ to the nation’s aviation heritage. Note that if an airframe is rare, or a one-off, this does not automatically make it important.
This heading is also used for the products of what is considered to be an ’endangered’ manufacturer or similar organisation, the survival of a representative of which is regarded as important.

F - Condition
This relates to the originality of the airframe, what was referred to as ‘museum-worthiness’ in the earlier Aviation Heritage UK document "Priorities for Preservation". Hybrids, as significantly reconstructed (as opposed to restored) airframes are rated lower than more original examples although this becomes less important with age/rarity and the number of ‘restorations’ an airframe has undergone. Likewise, account is taken of airframes or nose sections that are substantially incomplete or damaged.

The headings A to E, and in some cases F, refer to the status of an airframe prior to its inclusion in a museum or collection. The following two headings reflect the current standard of care for each airframe. By improving these standards, the prospects for an airframe’s long term survivability can be improved.

G - Museum / Collection

This recognises the airframe as a valued part of a collection being managed by an established organisation. This need not necessarily extend to a collection meeting the standards for full Registered Museum status, although this is encouraged as part of Aviation Heritage Uk's "National Aviation Heritage Strategy". What is important is that there is a clear organisational entity that can be held accountable for the protection of the airframe. As has already been states, airframes on loan frequently provide a ‘grey’ area relating to this responsibility. With this edition a start has been made to highlight airframes on loan to collections - it must be pointed out that this is far from complete and must not be considered as definitive.
Another point (*) can be added if the collection has a permanent workshop and/or restoration facility and a proven ‘track record’ for long-term planning.

H - Environment
This recognises that the airframe is normally housed undercover or maintained in an airworthy state. Note that an airframe moving from one museum to another may acquire or lose points under this heading.
Another point (*) can be added to recognise higher standards of care and conservation. This could include being housed in a controlled, air-conditioned environment, access to specialist staff etc.

I - Local Significance
This final heading, which applies only to specific airframes and not to types in general, recognises that where an airframe is preserved may, under certain circumstances, represent an added dimension.

This recognises where an airframe has been built or operated at, or near, the location of the museum or collection, or within the particular region covered by the museum or collection and defined as being of special interest to its collecting policy.

Assessment of each proposal is on its merits.

The Register relies considerably from inputs from the custodians themselves, particularly where a case for ‘Local Significance’ is to be made. Where necessary, the rationale for inclusion will be stated as a footnote. As elsewhere in the Register, an airframe moving from one museum or collection to another may acquire or lose points for local or regional significance.