View Full Version : Alt bust
Question came up in cruise on the way back from TFS as to how long you have to vacate a cleared level before it is defined as an alt bust. Obviously if you forget and half an hour later you are still there...but what is the maximum time allowed to vacate? Told my man I'd have the answer next time I flew with him.
Andy Mayes
24th Sep 2010, 22:46
How long? I think the question should be 'how much'. Mode C accuracy is +/- 200ft, go above or below your assigned level by more than that even for a second and its a level bust.
I understand that when we "squawk ident" one of the several things they are checking for is this 200ft tolerance.
I'm surprised you don't know this, we get this hammered home during sim's.
HEATHROW DIRECTOR
25th Sep 2010, 06:59
Squawk Ident has nothing to do with checking any altitude tolerance. Squawk Ident is to confirm the radar identification and produces a flashing circle, or similar symbol, on the aircraft label. The altitude appears on the radar so the controller can see it all the time but it is a requirement for pilots to report their altitude when requested so that ATC can validate the altitude read-out.
If ATC clears an aircraft to a different level it's up to the pilot when he complies. A level bust would not occur simply because the pilot was slow to leave a level.
Current controllers might offer other opinions?
Talkdownman
25th Sep 2010, 07:26
how long you have to vacate a cleared level before it is defined as an alt bust. Obviously if you forget and half an hour later you are still there...but what is the maximum time allowed to vacate?
I don't think there is one. If it is an unrestricted 'descend/climb when ready, report leaving' / 'pilot's discretion' type of vertical clearance then you can stay there all day if you like. You might be in the middle of a meal or a little doze. If you are forgetful that might be a bit of a problem for a pilot. Presumably you would wish to remember to land eventually before fuel expiry. The controller might even like to go home at some stage.
If you have an 'Expect to be level by/abeam' or 'leave now' type of clearance then that is an entirely different matter. I think with a 'leave now' clearance you would be expected to acquire the min 500fpm ROD/ROC as soon as reasonably practicable.
BrATCO
25th Sep 2010, 17:45
I usually expect an average of 1000' per min from the time I give the clearance. Except with A340s.
I mention it when I need more ("be leveled by...") and/or if less is acceptable("start at your discretion"). Both can be used in the same clearance.
In case of no prior "contract", if you haven't left your level 1 min or so after your clearance, I will call you back. Just to confirm.
If you wish to maintain your level, just say it. Sometimes, different solutions can be found.
Blockla
25th Sep 2010, 18:22
but what is the maximum time allowed to vacate?1 minute from receiving the clearance; it's not often enforced in terms of times up I'm "dobbing" you in... But it could be. In busy environments the controllers would always expect a better reaction time than a minute...
The mode C tolerance is 200 feet, the pilot tolerance is 100 feet.
zkdli
25th Sep 2010, 18:25
In United Kingdom Airspace the definition of an Alt Bust or in UK parlance a "Level Bust" is a deviation of 300ft or more from your cleared level. There is no time limit for the bust.:)
Regarding vacating cleared levels, it depends on the clearance. If the instruction is descend when ready XXX then you may remain at your current level until you are able to follow your chosen descent profile. Unlike the United States, with this clearance once the level change has commenced you must continue change and not stop at an intermediate level (I think that this is US procedure happy to be corrected)
With all level change clearances in the UK, you are required to vacate the level as soon as practicable after the issueing of the clearance and you must change your level at a mimimum rate of 500ft per minute. If you are unable to achieve this rate you are required to inform the controller. the exception is when making a continuous descent approach (CDA),when you vary your descent rate to match the miles from touchdown. :)
added stuff
TDK mk2
25th Sep 2010, 19:31
I had a technical level bust after a windshear go around a few years back. ATC didn't actually see it but I knew I had done it so ASRed it. Even one level bust can hugely skew the statistics for a small airline and the regulator starts getting very interested. Fortunately on this occaision a bit of 'liason' between the company and ATC revealed that we were only at 3300' for one sweep of the radar and the ATC manager decided that given the weather conditions (which caused the GA) he didn't regard it as a genuine level bust. The ASR was subsequently 'redrafted'.
My lesson; ATC are your friends...
OK guys thanks ...so here is my readback. Except for the obvious "descend now" (and whatever is required to meet an associated alt constraint) there is no time limit to vacate...but exceed a notional one minute and you can expect a reminder.
Andy M- I see you have only posted five times...here are a few dos and don'ts.
- Do RTFQ.
-Don't tell someone the question they are trying to ask ...if you haven't grasped what it is they are asking and why.
- Don't assume (incorrectly) people don't know the answer to your basic unrelated question.
- Having commited said sins...don't end with a smarta*$ "I'm surprised you don't know the answer to this" it just winds people up.
tow-bar
2nd Oct 2010, 08:48
Can you call it Level Bust if one ACFT is maintaining level 252 (FL250) and another one is on level 258 (FL260)?
5milesbaby
2nd Oct 2010, 09:17
Tow-bar - that is perfectly acceptable and does happen, so no, that isn't a level bust.
RMC
Except for the obvious "descend now" (and whatever is required to meet an associated alt constraint) there is no time limit to vacate...but exceed a notional one minute and you can expect a reminder.
Wrong. "descend now" isn't phraseology, its either "descend...." or "when ready, descend...." (or "at your discretion, descend..." depending on country and controller). Unfortunately, if we don't use "descend now..." when busy it gets a reply "is that now or when ready?" which is RTF we don't need. I don't apologise to anyone who received a "did I say when ready? go down now" as a response, its rather an annoyance as they must be able to hear just how busy the frequency is. So to answer your question correctly, if given a discretional descent there is no time limit to vacate. If given a normal descent clearance, you are to vacate as soon as is practical, and if you haven't within 30 seconds, expect a chasing call to confirm you are decending.
Andy M was right in one sense, a level bust is exceeding 200ft from your assigned level. If you fail to descend when instructed to do so, that is a failure to comply with ATC instruction.
320 or greater
2nd Oct 2010, 14:04
Totally agree with 5miles, as an UK controller it's either descend (meaning now) or descend when ready (at pilots discretion, bearing in mind the plates). Annoys when pilot inserts an unissued 'when ready' into the readback!
a level bust is exceeding 200ft from your assigned level. - you sure of that? NATS don't agree! Altimeters are only +/-200'.
5milesbaby
2nd Oct 2010, 15:35
BOAC, thats what I said, exceeding 200ft from assigned level, so +/-200ft or less is within the tolerance.
you sure of that? NATS don't agree! Altimeters are only +/-200'.
You mean the regulatory max vertical navigation performance, right? Because my 737 tells me its actual vertical navigation performance is around 150 to 180 ft at FL410 and getting better (smaller) below that. Required vertical navigation performance is set as fixed 400ft throughout the usable altitude band and can, unlike lateral RNP, not be changed by pilot input.
However for my company deviating by more than 150 ft is an altitude bust that has to be reported.
NATS say 300ft to 'qualify' for a level bust, not 201ft
A report citing a perceived "level bust" may be filed when the mode C indication that the controller sees indicates that the flight has reached 300ft or more from the cleared level.
ATC only see mode C indications and they are in hundreds of feet.
The aircraft may be maintaining FL250 but mode C may indicate FL248 and ATC will be happy.
If that aircraft climbs to FL254 (busts their level) ATC will see FL252 and will not have a problem.
If that aircraft descends to FL249, ATC will see FL247 and will make a report.
In the last case there was no "level bust". However, ATC are required to check and report.
Aircraft altimeters have allowable errors.
Mode C encoders also have allowable errors.
Pilots are expected to fly to a certain tollerance.
If all 3 errors line-up, the level indicated at ATC can be well outside what they expect despite the fact that pilot pilot is (just) within the allowed tollerance.
Remember that separation standards (vertical as well as horizontal) are designed to prevent collisions - i.e. the flight's don't hit. They are not designed to keep a minimum of for example 1000ft vertical distance physically between the flights.
BwatchGRUNT
4th Oct 2010, 13:27
In 12 years of ATC I have never wtnessed a level bust as a result of failing to set standard QNH but yesterday I saw it three times on bristol departures. Due to the low pressure this was a deviation from the flight level of 600-700', lots of paperwork should have been completed - no harm done fortunately so guess it was a lesson learned for them anyway.
One biggish UK carrier, one biz jet on a repositioning flight and one big European carrier!!
5 Miles - just got back from a long weekend and saw your.
"I don't apologise to anyone who received a "did I say when ready? go down now" as a response"
I have no time for pilots who think it is macho to talk to controllers/ cabin crew or anyone else in that fashion. Crew Resource Management was introduced as a result of accidents (Kegworth et al) where that kind of parent child attitude was identified as a major link in the chain....without it many more people would be alive today.
Yes this is at the extreme end of the spectrum, but whichever way you cut it that statement has no place in aviation.What do your procedures say about making condescending comments over the airwaves?
bwatchgrunt did you just say that you did not file three MORs that are required under CAP382?
tow-bar
6th Oct 2010, 14:15
But on the other hand ATC must provide 1000' vertical separation
ToweringCu
6th Oct 2010, 14:55
If one is at fl250 (indicating 252) and the other is at fl260 (indicating 258) then the controller still has 1000' separation, because both transponders are within tolerance.
Mode S supports an altitude resolution of 25 ft, is that part of the required mode s standard in core europe? Not that it really matters at high altitude.
Surely the need for an altitude encoder is the governing factor for accuracy, hence +/-200ft?