Hi All,

I have recently been trawling through my ERSA trying to understand everything before by PPL flight test and I am confused about the Aerodrome Reference Code (CN) in the RDS. The part that gets me is the "This code number indicates the maximum field length of the aeroplane that the RWY is designed for" (INTRO 23-p.41).

What is a field length? And why do we need to know this information?

Any help would be much appreciated

Locky27

Hi Locky,

You don't need to worry about field lengths in the exam as far as I know.

The aircraft reference field length of an aircraft is simply a value assigned to the aircraft based on its performance figures at certification. With this information, the pilot can look at the CN for a runway to determine whether the runway is going to be theoretically suitable for their particular type.

For example, if you are flying an aircraft with a field length of 1300m it means, at certification, the powers that be determined the aircraft required a field length of 1300m. This is not the actual length you would need on the day but rather a standard performance figure assigned to that aircraft type. I imagine it would be safe to assume your actual performance on the day would never, under normal circumstances, exceed the field length assigned to your aircraft type.

A runway with a CN of 2 (for example) has the appropriate approach/take-off gradients and surveyed areas for aircraft with a "field length" of up to 1200 m. With our field length of 1300m, we can immediately see that runway may not be able to provide us with the legal requirements for obstacle free gradients etc. A runway with a CN of 3 or 4 should theoretically pose no problems.

As the ERSA states though, these are not meant to limit aircraft operations at an aerodrome. They are not saying you must not use a runway with a CN below your field length. They are just saying it wasn't designed for an aircraft with your field length. You are still free to use it if your calculations show you can do so safely given the conditions of the day.

Similarly, just because the CN says the runway should accommodate aircraft with your field length, you are not excused from making appropriate performance calculations relevant to your take-off or landing.

Cheers,

Rich