Mode S – the CAA Decision on Carriage July 2009

The CAA has now published its decision on Mode S effective 1 October 2009.

You can read the CAA decision letter here but this summary for light aircraft owners and pilots has been checked for us by the CAA.

Please note the following:

The AIP says that transponders must be carried in particular circumstances and the means of compliance with that is Mode S unless you already have an exemption that allows you to use Mode A/C instead until valid until 31 March 2012.

The Rules Are:

If you fly only VFR below FL100 you do not need Mode S in your aeroplane.

If you have a Mode A or A/C you can continue to use it as long as you wish.


If you fly for public transport anywhere you must have Mode S.

If you fly IFR or VFR in Class A, B or C airspace you must have Mode S.

If you fly IFR in Class D or E airspace you must have Mode S.

If you fly in a TMZ you must have Mode S unless:

xxxxxxxxxYou obtain a clearance from ATC or,

xxxxxxxxxYou comply with the published procedures for aircraft without a transponder.

The CAA will not approve a new installation of Mode A or A/C so Mode S is it.

The CAA will approve existing installation upgrades from A to A/C.


This means that you do not need Mode S:

To fly VFR in Class G airspace

To cross Class D airspace (with clearance)

To fly in a TMZ (with clearance or following approved procedures)

To fly internationally

However .....


You will find clearances easier to get if you have Mode A/C or fit a Mode S and if you are flying in a radar service area it will also help reduce the risk of collision and the consequences of an inadvertent airspace incursion. 

If you have a transponder, please turn it on to ALT as part of your pre-take off checklist.  Why don’t you add that to your checklist now?

5th August 2009