ATC warning about height on approach

[Mr Optimistic]

The thread title no doubt shows how little I know, however given that we have had two avcidents recently where aircraft have been well below the appropriate height late in the approach, why is it that ATC dont warn? They see a transpondered height dont they?

[Ka6crpe]

Possibly because ATC are too busy managing the traffic flow to do the pilot's job as well.

Seriously, I would hope that ATC are looking after seperation and leave it up to the pilot to fly the aircraft. Maybe what is needed is for pilots to know how to actually fly rather than how to operate a computer.

[HEATHROW DIRECTOR]

Some towers do not have radar or, if they do, it may not have SSR so they may not know if the aircraft has descended too low.

[Mr Optimistic]

Thanks but SFO and Birmingham ? Assume it was quiet in the early morning at the latter.

[thing]

As K6 says, it's up to the pilot to fly the aircraft, ATC have other things to do besides monitoring approaches. Apart from that a transponder doesn't necessarily give height above the ground, so the height on the ATC guys display may be totally meaningless in terms of approach height.

If you were flying a PAR approach then fair enough, you do what ever the guy watching the tubes tells you but AFAIK they don't do these approaches at major airports although I'm willing to be shot down on that one.

PAR is certainly still in use at mil airfields.

[Level bust]

When we did our TRUCE training on the SIM, we were all TWR only, one of the 'incidents' we did was descending below the glide path.

It got controllers to think about that scenario, and yes the mode C on the ATM does show height AMSL but every ATCO should know how high their airfield is, and take that amount off the mode C!

[thing]

and yes the mode C on the ATM does show height AMSL

No it doesn't, it can be quite a bit in error and still be within tolerance that's acceptable to ATC. I was crossing Donny zone the other day and my Tranny was indicating 4,000 when I was at 3,000 It gradually sorted itself out to 3,300; the controller and I were speaking about it and he said that it was within tolerance at 3,300.

I have no idea what the tolerances are but obviously 10% at least.

Mode C is like a stopped clock, right about twice a day...

Even an encoding alt depends on having the right QFE set. Errors (I've done it myself) come when you're given ones and noughts, IE 'QNH 1011' can be easily misread as 1001, you may even read back 1011. That's 280' of error for a start.

[HEATHROW DIRECTOR]

Tranny... Donny? Don't understand your banter OM!

Sounds like your transponder was at fault, which in my expereince was not unusual with light aircraft. Commercial aircraft transponders are pretty good and if I had seen a steady 10% error on an aircraft in level flight I'd have got a bit nervous.

[thing]

Transponder and Doncaster Class D OM!

Just out of interest what are the tolerances for Mode C (non encoding alt)?

[Burnie5204]

Given that (I'm gueesing it's the PAR system) mil controllers are able to get a 'side view' of the approach showing the aircraft position on the glideslope (range and height) could a similar system not be used in 'civilian' airfields with an alerter buzzer if an aircraft drops below a pre-set lower limit? Would it even be possible I suppose would be the next question.



thing - working at EMA I've heard our Radar/App controllers talk to GA A/C with Mode Charlie inaccurancies talk about the Mode C tolerance being required to be within 300' of actual altitude

[DaveReidUK]

I was crossing Donny zone the other day and my Tranny was indicating 4,000 when I was at 3,000

It's perfectly possible for your transmitted altitude and actual altitude to differ significantly.

Mode C and Mode S transponders always send altitudes relative to QNE (1013.2 hPa) whereas at that height you will (hopefully) have had your altimeter set to QNH or QFE.

Even an encoding alt depends on having the right QFE set.

No - see above. What you see will depend on your baro setting. What the encoder sends to ATC doesn't (though the controller will see the correct altitude because the ATC computer will adjust it for the known QNH).

[thing]

Dave

There wasn't a thousand foot of QNH/QNE difference, QNH was around 1014 as I remember. I've flown a few different GA aircraft with similar transponder inaccuracies. You know what's coming when ATC say 'G*** repeat altitude?'...

I weas under the impression that the big advantage of an encoding alt was it sent the mode charlie corrected for whatever you had set on the subscale. I'm not a tech head so I'm probably wrong and stand to be corrected.

[LookingForAJob]

working at EMA I've heard our Radar/App controllers talk to GA A/C with Mode Charlie inaccurancies talk about the Mode C tolerance being required to be within 300' of actual altitude

Not strictly true. I can't recall the accuracy requirement for transponder altitude reporting but the 300ft that you've heard relates to the ability of a controller to accept variations in the reported level of an aircraft for separation purposes. These variations result from a wide variety factors including transponder/encoding altimeters becoming less accurate between maintenance and pilots/autopilots not flying at the precise level assigned. The relevant ICAO book, PANS-ATM, says:

8.5.5.2 DETERMINATION OF LEVEL OCCUPANCY

8.5.5.2.1 The criterion which shall be used to determine that a specific level is occupied by an aircraft shall be ±60 m (±200 ft) in RVSM airspace. In other airspace, it shall be ±90 m (±300 ft), except that the appropriate ATS authority may specify a smaller criterion, but not less than ±60 m (±200 ft), if this is found to be more practical.

8.5.5.2.2 Aircraft maintaining a level. An aircraft is considered to be maintaining its assigned level as long as the pressure-altitude-derived level information indicates that it is within the appropriate tolerances of the assigned level, as specified in 8.5.5.2.1.

8.5.5.2.3 Aircraft vacating a level. An aircraft cleared to leave a level is considered to have commenced its manoeuvre and vacated the previously occupied level when the pressure-altitude-derived level information indicates a change of more than 90 m (300 ft) in the anticipated direction from its previously assigned level.

8.5.5.2.4 Aircraft passing a level in climb or descent. An aircraft in climb or descent is considered to have crossed a level when the pressure-altitude-derived level information indicates that it has passed this level in the required direction by more than 90 m (300 ft).

8.5.5.2.5 Aircraft reaching a level. An aircraft is considered to have reached the level to which it has been cleared when the elapsed time of three display updates, three sensor updates or 15 seconds, whichever is the greater, has passed since the pressure-altitude-derived level information has indicated that it is within the appropriate tolerances of the assigned level, as
specified in 8.5.5.2.1.

8.5.5.2.6 Intervention by a controller shall only be required if differences in level information between that displayed to the controller and that used for control purposes are in excess of the values stated above.

It's not correct either to suggest that 'transmitted altitude and actual altitude to differ significantly'. Maybe I'm being a bit pedantic but a transponder that is working properly will always report a flight level - and the reported level will correctly reflect the FL equivalent of the aircraft's altitude. As has already been mentioned, ATC radar systems usually convert the FL reported by an aircraft to the equivalent altitude when the aircraft is below the transition altitude.

[thing]

Excellent LFAJ, answered my tolerance question precisely, thank you. Every day is a schoolday on Pprune!

[DaveReidUK]

I was under the impression that the big advantage of an encoding alt was it sent the mode charlie corrected for whatever you had set on the subscale. I'm not a tech head so I'm probably wrong and stand to be corrected.

No, it's the ATC computer that performs the correction. All that the encoding does is to convert the raw, uncorrected altitude into the appropriate value (e.g. Grey code) and pass it to the transponder.

Having said that, nowadays aircraft equipped with Mode S EHS do in fact send the pilot's selected baro setting as one of the downlinked parameters, though I'm not sure what (if anything) the controllers do with it.

[thing]

Thanks for that.

Another question then that you may be able to answer. I'm often asked for my altitude on a certain pressure setting, is that so they can fiddle with their display to make it read the same as I'm reading?

[DaveReidUK]

I don't know.

Possibly to avoid you becoming a CFIT statistic ...

[HEATHROW DIRECTOR]

<<I'm often asked for my altitude on a certain pressure setting, is that so they can fiddle with their display to make it read the same as I'm reading?>>

They can't fiddle with their displays. I imagine they want to obtain your altitude on a pressure setting being used by other traffic they are working?

[thing]

Could be. You know when you've been crossing CAS on their QNH, when you leave CAS they give you the regional which invariably different to their QNH, they'll often ask for altitude on the new pressure setting, just wondered why.

[reportyourlevel]

Mode C and Mode S transponders always send altitudes relative to QNE (1013.2 hPa)

I think you need to check your definition of QNE.

They can't fiddle with their displays.

Yes we can.