Yep, that's about it.

G

Lets try this again. You do not need a transponder to fly in a TMZ. If you do not have a transponder you request permission to enter by radio, which is almost always a formality. Alternatively, you can operate under a letter of agreement. In this case you would be based or visiting an airfield inside a TMZ and would obey the procedures in the agreement with no need for radio contact. It is perfectly possible to routinely operate in a TMZ with no transponder and many micros do this all the time.

Rod1

It's the way you phrased it Rod; it could give people the impression they can transit a TMZ with no transponder and without calling on the notified transit frequency, whereas in actual fact, you do need either radio contact or an LoA or preferably both. I presume you're referring to Hunsdon when you mention microlights, in which case it is possible they do have an LoA with Stansted.

Quite a lot of people responded very vigorously to Stansted's consultation when they originally proposed a TMZ. The main reason was that it was seen as a precedent and any flight privileges lost in Class G airspace at Stansted could easily be repeated around the UK. The gliding community is particularly opposed to mandatory equipping of gliders with transponders because of the lack of a suitable power supply in most gliders, and other arguments that I won't bore you with.

Apart from a reduction in size of the proposed TMZ (Stansted probably asked for more than they wanted so they could get the smaller areas actually awarded) the most important concession won was that aircraft without transponders would be permitted entry if radio contact is made before entry. This is not 'permission' to enter. A pilot only needs to ask for a basic service and state his intention to enter and remain in radio contact until clear. (Sites with Letters of Agreement may have different rules.)

The full details are published by the CAA. In summary:
In order to operate in the Stansted TMZ, it is proposed that aircraft will be required to operate a fully functioning pressure altitude reporting transponder or comply with the following access arrangements:

• Inbound and outbound procedures published by specific GA airfields and in accordance with any Letter of Agreement that the airfield has with Swanwick Terminal Control.
• In accordance with flight details passed to and an acknowledgement from Farnborough Radar/Essex Radar.

So the illogical position that transponders are not needed in TMZs is one that was won after many representations to the CAA and others.

I'm sorry, but I think such behavior is just selfish.

A lot of busy airports in the US have a full TMZ around it. The mode-C transponder is essential for keeping everyone safe, including yourself, when operating under or within the terminal airspace.

One can also get a clearance (at the controllers discretion) when you don't have a transponder.

I think there is a very substantial selfish element in all this - often presented as a "civil liberty" / "government surveillance" case.

There are aircraft types which have problems with carrying transponders but the majority are normal powered types with a normal electrical system and they could easily install a Mode C.

Unfortunately N Europe has screwed up badly on the PR on this, mandating Mode S but with a totally useless technical case (basically, Mode C works just fine for TCAS purposes) which undermined the campaign for transponder adoption.

So we now have a situation where TCAS is a lot less than effective, a radar service from ATC is nearly useless (the vast majority of traffic they report is "altitude unknown") despite the millions spent on providing it (the powers to be pay for it only to keep a lid on CAS busts; not as a service to GA which pays no enroute fees) and the most effective way by far to minimise the chance of a midair is to (a) fly in IMC; (b) fly at weird numbers like 3300ft, 3700ft, 4300ft, etc; (c) fly above 2000ft absolutely whenever at all possible.

So, I'm going completely off subject (but hey, its my thread, I can!), but how much power does a transponder draw, and why don't they look at two levels of system?

The equivalent (as I understand it) from a sailing perspective is AIS, which has two flavours. One has full information and is always on (basically for large powered vessels which don't have things like power draw issues) and the other which polls intermittently every 30s, and provides reduced info. This draws a lot less power, and tracking systems are designed to take this somewhat "old" data into consideration, effectively generating tracks, but with an element of uncertainty.

It seems to me that transponders are a great idea, but for the sake of a relatively simple solution, are barred from aircraft without engines (and so no independent power source) or aircraft that can't take the weight of big batteries (hence, the polling suggestion).

Or is there something more fundamental I am missing?

IPZ

A transponder is different to a transmitter - it responds to being interrogated. So you have to have it on all the time to be useful.


A transponder installed typically needs around 250W at peak, but I'm not sure how that translates to mean consumption - certainly a lot less than that, but still enough to drain a lightweight battery in well under an hour.

G

A GTX330 draws about 0.5A at 28V i.e. about 14 watts.

Doubling the current draw for 12V, one of these (weight 2.6kg) would last about 6hrs.

The biggest problem with fitting transponders is regulatory, not technical. We have done this debate many times.:ugh:

Rod1
(I have mode C and I use it)

The airspace around Stansted that is used by its commercial traffic is already controlled airspace Class D mainly from the surface to 2,500' (from 1500' under the stubs) and above that it is Class A where VFR flight is prohibited. Because the Class D zone is very busy light aircraft etc. are rarely granted entry. The TMZ applies to uncontrolled airspace that is not used by Stansted's commercial traffic and only goes from the surface to 1500'. I think the situation is very different to the one you are comparing it with in the USA. The orginal proposal was for a larger TMZ - again covering airspace not used by Stansted and within which they do not offer any sort of air traffic service.

I don't accept that is entirely true - otherwise the CAA would have made them mandatory for everyone when they originally proposed doing so, but I'm not going to bite any deeper. As you say it's been done to death many times in the past.

More 2nd-hand information from the NATS man who I listened to a couple of months ago. (And more thread creep, too, but I think it's interesting and almost relevant!)

He was involved in creating the TMZ, and he pointed out the fact that it is circular. This is a major change in UK airspace policy. Traditionally, he explained, airspace boundaries are very complex, because of pressures to allocate only the exact amount of airspace that is required. This complexity, he argued, makes it difficult for pilots to understand and avoid. Look at the north-west corner of the Southampton zone for example - I reckon I know that bit of airspace pretty well, but I still have to study the chart pretty closely to work out exactly where the boundaries are between each of the many different bases.

As well as complex shapes, this policy also means that airliners fly very close to the edge of airspace, and therefore very close to unknown aircraft with unknown intentions outside of controlled airspace.

What NATS would like, according to this chap, is for more airspace to be defined in much simpler shapes, such as the circular TMZ, and for it to be considered normal for pilots to request, and be granted, access to this airspace. Granted that this means there will be more airspace than the current system, but if access to this airspace if freely available by making a request on the radio, the privileges of GA aircraft will not be affected, the simplicity will help avoid airspace busts, and controllers will be talking to all of the aircraft who are flying very close to their airliners with all the safety benefits that go with that.

He explained that the Directorate of Airspace Policy was very much against the idea of the TMZ being circular, but NATS won this argument, and hope that the ease of access to the TMZ will set a precedent, and encourage DAP to allow future airspace to be defined in a similar way in the future.

FFF
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Can't have that. They have that in the USA. It thus cannot work in Europe :) :) :) :)

A fantastic idea! :ok: Is he American? ;)

Jim59:

Over in America it is quite normal for someone flying VFR to be granted clearance to fly through most terminal airspace, but as priority is (understandably) given to IFR aircraft and aircraft landing and departing the primary airport of which that airspace is designed to protect, you can also get refusals for Flight Following or transit during its peak time.

AFAIK there is no difference in priority whether an aircraft is commercial or non-commercially operated - everyone's safety and lives are worth the same - although the heavys will again understandably be given more attention.

Clearance or no clearance, operating inside, above or beneath the shelf of busy Class C and B airspace generally does require a mode-C transponder, which I think is very reasonable and certainly the safer option. It gives ATCers and other pilots equipped with traffic alert systems much less headache!