Stansted TMZ Airspace Change Proposal – LAA update 9th April 2009


Consultation closing date delayed to 17 April 2009 (extended by 10 days)


NATS has proposed that the area under the Class D Stansted TMA (the stubs of the end of the runway and the wings either side of the CTR) be designated a Transponder Mandatory Zone (TMZ).  If you have an operating altitude encoding transponder you would be free to fly in the area without restriction but if you don’t you would be excluded except under special arrangements (see later).


You can see the proposal here.  The proposal does not set out any access arrangements but following negotiation with some users NATS have proposed access to some areas which you can see here. This means that any aircraft with a radio would be able to fly under the stubs but on aircraft with transponders would be allowed to fly under the wings.  There appears to be less risk in the wings and the areas is less critical for commercial traffic.


The CAA has said that it will consider establishing transponder mandatory zones where there are incursion hot-spots near controlled airspace and it will use the standard Airspace Change Process (ACP) to consider proposals.  We do not challenge that.


The basic premise of the NATS proposal is sound; ATC will be able to manage airspace incursions better if all aircraft flying under the TMA carry altitude encoding transponders.  They will be able to tell if an aircraft flying under the TMA is really below it or if it has inadvertently climbed into controlled airspace.  Indeed, modern ATC systems can automatically warn controllers if that should occur and highlight any possible conflict.  If an aircraft without a transponder enters the TMZ, ATC would presume that it was actually in the TMA above it and take avoiding action.  At the moment, any non-transponding aircraft seen in the area under the TMA is presumed to be outside controlled airspace and ignored.


That is a statement of a situation that can be applied to all controlled airspace that has class G airspace underneath it.  If carried here, the same logic would be applied under all controlled airspace in the UK except CTRs which are always established from the ground up anyway.  About 40% of the land area of the UK has controlled airspace over it so that would be prohibited to non-transponding aircraft.  The proposal says nothing about extent and management of risk in this particular airspace. 


In this proposal NATS does not tell us about the incursion risk at Stansted.  Indeed they go to considerable lengths in the proposal to hide the figures.  We have asked them over and over but they refuse.  The cynical amongst you (me included) will think that NATS does not want you to know the figures because they do not support their case.  But incursion risk is the only reason for establishing a TMZ.  NATS disguises all the data by giving various percentage figures but not actual data so we cannot check it.  Using data from other sources we have discovered that there were only 3 high risk incursions in 2008 in the whole of the UK.


We propose that if NATS had given us safety data which supported the proposal, we would probably have agreed to the TMZ under the stubs but would have argued against the wings. NATS proposes good access to the stubs but not the wings see the NATS Update here.  However, we have heard that NATS has told the CAA it now wants to be paid for the provision of services to aircraft outside controlled airspace and that may include clearances to enter the TMZ.  We have asked NATS to confirm or deny that.


You can see the LAA draft response HERE.


Please send NATS your views on this consultation by email to