If you wish to contribute or participate in the discussions about articles you are invited to join SKYbrary as a registered user

 Actions

Level Bust with more than 1000 ft Safety Alert/Response

From SKYbrary Wiki

LIST OF FULL RESPONSES

1. This is not an easy question.

Currently the majority of national military (OAT) rules require returning to the assigned altitude/FL immediately. Normally, a level bust with a military high performance jet is very rare, since there is great emphasis placed in training and annual proficiency check flights on correct level-offs. Special procedures are in place to break climb-rates of 6000 ft/m and more, e.g. cutting the climb-rate in half at least 1000 ft prior level-off.

However, an additional procedure as proposed below may be a viable measure to increase safety, but only if conducted on a pan-European scale and preferably applicable for all aircraft (civil and military under GAT and OAT). Otherwise there would be a undesirable mix of procedures that could rather worsen the situation.

On the other hand, a high performance jet has returned to the assigned level after a level-bust normally faster than the ATCO realises it.

This issue surely requires some expert-level discussions.

By the way, all military aircraft can generally be contacted on an emergency frequency.


2. Simple solution could be handled with clear concise phraseology:

"Control, ABC123 Level Bust FL90 maintaining" The important words being the Level Bust statement giving information that the aircraft is no longer at the cleared level and something needs to be done about it..

As a controller that would get my attention pretty quickly and would be able offer avoiding action as an immediate response.

The response thereafter would be

"ABC123, Control Level bust acknowledged, instruction which could be turns, climb or descent". If time permits, then "ABC123 request intentions"

Obviously TCAS may well handle some of the immediate advice, but with the phraseology above everyone is alerted to someone being somewhere they shouldn't be.


3. Here is view for a former military controller who spent half his 30 year military career in joint en-route civil/military operations.

Military Pilot should, upon realising that he has bust the cleared level by more than 1000ft shall:

- Level off at the closest intermediate 500ft level (this should ensure that the aircraft does not bust another flight level); - Transmit: LEVEL BUST, LEVEL BUST, LEVEL BUST, (Aircraft Callsign) now level at (for example 2500ft/FL115)

It will be interesting to hear what other reactions you get.


4. Answering your Request "Is there a need for a procedure or a recommendation for pilots in this kind of level bust incidents? How could the possible solution look like?"

In the actual case, a military fighter aircraft has normally very high rates of climb (power/weight ratios) and descent. The high ROCs may make it easy to level bust, and this is compounded when flying in "civilian" airspace where there are normally various aircraft in the same area.

Our Standard Operating Procedures (SOPs) call for a confirmation by both pilots prior to any change of altitude/level requested by the ATC the actual altitude or level.


5. I think for a commercial airplane is difficult to have a level bust of more than 1000 feet. So, in case it happens, I think a good recommendation is to level off at the closest 500 feet step.


6. As far as I know, we were not confronted with an level bust you described. Nevertheless I think the proposed solution is a safe way. Keep a 500ft emergency separation and proceed after an appropriate clearance.


7. With a 2 pilot crew having a >1000ft altitude bust should be a very rare occurrence. I am guessing that ATC would be calling you before that. At that point the most prudent thing to do would be to return to the properly assigned altitude as soon as possible unless of a TCAS conflict or a new assigned altitude by ATC.

I believe the likely cause of this would be a misinterpreted assigned altitude and something causes confusion in Europe is the fact that there is no standard altitude for setting QNH. This is a large distraction. It even affects how you interpret the altitude assignments. ex. Flight level 60 or 6 thousand feet. From a human factors perspective this is unnecessary risk.


8. A very interesting subject; as a pilot myself I tend to agree that on initial recognition of a level bust you would immediately try to recover to your cleared level which as the article mentions could be detrimental. I agree with the solution provided that pilots climb or descend to the closest 500ft.


9. I have discussed your question with our Flight Crew Training Manager and concluded the following:

‘This is more concerned with high performance aircraft. However if this did occur with one of our B757, the aircraft would hopefully be picked up by ATC, if not we would expect the crews to contact ATC and tell them that they are returning to the cleared level and leave it to ATC to advise whether that is the best course of action or not. After all, if the crew had lost communication as well that is what ATC would expect them to do?’


10. Limit rates of climb and descent to a maximum of four thousand feet per minute inside TMAs. This restriction will only affect very high performance aircraft such as military fighters and certain business jets and thus it will have minimal effect on GAT. By limiting the rate of climb and descent, there will be more time for a pilot and an air traffic controller to react to any level busts.

In case of a level bust of more than 1,000 feet, the pilot can level off at the next intermediate level. In case of busts during climbs, the pilot is to level off at the next intermediate level above the present level. In case of busts during descents, the pilot is to level off at the next intermediate level below the present level. In order for a pilot to safely conduct this procedure, the rate of climb or rate of descent has to be contained otherwise structural stresses/damage and/or discomfort to passengers will most likely occur.

Thus, both items have to work together in order for a safe procedure to take place.


11. The question that I would like considered is whether granting another option will add confusion at a time when the pilots are already potentially in a situation where loss of SA has occurred (when the error is made by the flight crew this is more than often the case).

Given that these cases are rare (Alt busts are normally up to 500 ft anyway in my experience of our data) - surely TCAS will protect against the more dangerous case of passing through at least one other 1000 ft level? As long as TCAS is followed this will be safe.

This means that the crew can focus on where they are supposed to be and react to TCAS if required. My view is keep this simple and keep it to what is trained and understood - it is likely that most pilots would never see an alt bust of 1000 ft either by them or by another offender, so the course of action other than trying to get back to the cleared altitude would be very unfamiliar in the immediate circumstances.


12. I totally agree with 11. Both the intruder and the "victim" may be confused and decide to apply this additional separation. If it occurs, the risk of collision will be increased by a coefficient of 4. We trained pilots to follow TCAS-RA instructions regardless of the circumstances and it seems to be simple and effective.


13. I have discussed the proposal of a level bust solution (to change level to the next closest 500ft) with our Flight Safety Manager.

We have come to the conclusion, that proposed procedure is a logic and feasible solution of a level bust situation. We recommend accepting it as a standard recommended procedure for situations when there is no TCAS information or RA which needs different action. TCAS has of course the highest priority.


14. TCAS-RA at cruising altitude is a rare occurrence and we should strive to keep it that way or even less. However, RA`s on departure and specially on approach have dramatically increased during the last year and we all should make an effort to lower this undesired trend.

My opinion is that introducing new procedures to pilots operating in today’s complicated and crowded airspace may actually add to more confusion and would further complicate the situation.

Keep it simple and clear. Follow current procedures and TCAS commands, then communicate the situation to ATC as soon as possible and follow their instructions if not in conflict with TCAS. Do not assume that visual separation from traffic is safe enough. Their TCAS systems do not understand what you are trying to do but instead what you are actually doing.


15. I agree with 8, and with the original proposition of a 500' increment being a good compromise. Naturally, however, a TCAS Resolution Advisory (RA) should be followed regardless of this "500 ft" proposal.


16. Whilst I understand why the suggested solution of temporary level off at the 500ft ‘intermediate level’ for traffic which “(recognizes) a level bust by more than 1000ft by themselves” has been suggested, to adopt it would be most unwise I will attempt to explain why I hold this view by reviewing the factors involved - I assume that the DFS case involves controlled airspace.

(1) High rates of climb and descent are, as pointed out, almost exclusively a military fast jet issue.

(2) These aircraft often do not have ACAS fitted if they are manoeuvring and/or using high vertical speed then their Mode ‘C’ may generate a succession of ACAS RAs for the other aircraft involved which change too rapidly for it to be possible to follow them. Fortunately, the evidence is that such scenarios occur when the ‘offender’ has seen the ‘target’ and is avoiding it so collision is not going to occur.

(3) The principle that ATC control traffic separation until ACAS RAs intervene and these then take priority is a simple rule that must not be altered when even with the best ATC radar refresh rates are currently several seconds behind 1 second ACAS updates. ATC cannot actively manage close conflicts when they do not have real time traffic displays.

(4) It is vital that conflict resolution is to a ‘model’ that all parties understand and that there is no relevance in the model to which party has messed up. This is the model we currently have in controlled airspace with a civil mandate for ACAS.

(5) The present ACAS model covers cases where both aircraft have functioning ACAS (normal) and the case where one has only Mode ‘C’. In the controlled airspace case, this is sufficient.

(6) The case of military fast jets involves a number of points: • Historically, the cause of fast jet/civil conflict in controlled airspace has more often been airspace infringement by the fast jet than failure to follow an ATC clearance • Fast jets can usually see conflicting traffic on their radar early enough to avoid collision (but not early enough to avoid generating real concern for both the civil pilot and ATC • ACAS is not designed to manage traffic conflicts involving high performance military aircraft • Two fast jets in a controlled airspace conflict not identified by ATC would have to resolve it themselves - no real problem given on their on board equipment.

(7) A level bust is not sufficiently hazardous on its own, given the presence of ACAS on at least the ‘victim’ aircraft, to require a pilot-determined response rather than an ATC one.

(8) There is no way that safety can be enhanced by introducing a procedural action in controlled airspace which is based on the recognition by one of the potential three parties in a potential conflict that they have made a mistake and will therefore take an action which ATC will only find out about later.

It would certainly be possible to introduce vertical speed limits for all non-emergency traffic in controlled airspace and this would ‘contain’ vertical speeds of military fast jets but given the reasonable military expectations of FUA and the absence of a demonstrable safety case (as opposed to the understandable ‘concern’ of DFS, I do not see yet see why that could be justified. However, I think that there is most certainly a case for introducing mandatory vertical speed limits in controlled airspace for non military aircraft which will ensure that their ACAS can cope, especially in the (usual) case of co-ordinated RAs because civil aircraft do not have. VLJs might add to the risk already present with some business jets and empty commercial jets.


17. I tend to believe that we want to avoid adding confusion by adding further procedures but we need to open up very close discussions with the military, which I realize is not very easy and of course with the operators of the high performance executive jets including the up and coming VLJ, which I believe must be mandated to carry TCAS.


18. I think that level off at the closest 500 feet would most likely be the safest solution to this problem.


19. I would say that what 11 say make most sense. Since we also train all our pilots to follow TCAS at all time, as suggested, this should be enough to maintain the safety needed when an altitude bust of this size has occurred. To add additional procedures to this will, in my opinion, just complicate things.


20. We have the opinion that an additional procedure is not necessary. If a level bust of more than 1000’ occurs, it is up to the Captain to take appropriate action. This may result in a return to the original level, however, is other traffic is close, the return can be postponed, and all is concert with ATC.

TCAS is the last line of defence.


21. We have discussed the scenario internally and our view is that following altitude bust in excess of 1,000 ft the pilot should fly at a mid altitude (500 ft) until positive clearance is received to return to the assigned altitude. The only exception would be when responding to a TCAS RA when the TCAS commands should be followed.


22. Analyzing level-busts in our company show that the majority of them are caused by misunderstanding of controller's instructions, meaning that the pilots are unaware of flying in wrong levels. In the few cases where the level-bust was caused by mis-handling of the aircraft, it was recognized 100-300 feet and quickly corrected.

Therefore, I think that in the very rare case you describe, it should be in the judgment of the pilot how to react, according to his situational awareness, the communication and relying on TCAS.


23. When recognising a level bust, because of a TCAS RA or for any other reason, the pilot will be stressed. The simple and unambiguous advice should be to return to the authorised level ASAP. Should a TCAS RA intervene, then that should be followed until " Clear of Conflict" and then the pilot should return to the authorised level.

An additional procedure for very unusual large (> 1000ft) level busts may well cause additional flight deck stress and confusion.


24. The response from our side basically is the following:

It should be determined whether this is a structural problem or not. Based on 1 incident it is a little premature to start developing procedures. If, however, it can be determined that there is a structural problem, then the proposed use of an intermediate level could be considered as an appropriate solution. Contingencies on the North-Atlantic tracks use the same thinking.

The above having been said, a first consideration should be given also to the avionics kit onboard fighters that tend to give them the ability to identify all aircraft with transponders. With this kind of equipage it would be reasonable to expect that the military would assume the responsibility for the separation of aircraft. This would especially be the case in VMC as the fighter plans have a greater ability to be back in their box in a very short timeframe. It really is a case of having the military take responsibility and liability for keeping of their military training flights.


25. From our viewpoint, this proposal could increase the severity of the occurrence made by a level bust.

1) the AOT, which is mainly controlled from Defences’ facilities, operates the even and odd flight levels (examples: Fl 115, Fl 125) in all our airspace (included the RVSM area). The incident will affect two different control centres and will generate more confusion.

2) Within the ACC, some of these levels which are supposed to be free of traffic represent the limits between the lower and the upper sectors; therefore the traffic which is supposed to remain in the sector is not represented in the other sector. The chaos will affect two sectors instead of one.

Our proposal is that the States aircraft follow the same rules/regulations than the others airspace users, and that all this fleet be equipped as soon as possible with TCAS.


26. Views from some of our fleet managers are as below:

1) Rate of climb and descent has to be limited into RVSM airspace and into TMA and such information shall be in the sop (example limit your V/S to 1000ft/min once traffic is considered as threat

2) Strict adherence to TCAS is very essential and i don't recommended to adopt any new procedures as this will increase the load on the pilot

3) Adherence to sop especially standard calls


27. I checked our Database for level busts. Most level busts reported are less then a few hundred feet with an immediate return to the cleared level and almost all of them happen due to turbulence.

Others happened in countries using meters iso feet due to misreading of conversion info and misunderstanding of ATC.

In those countries the "500 feet rule" as a safe emergency level would be no help, since pilots usually don't know, what the normally used levels/heights are, once they are confused.

I support the idea of the max climb/descent rates within TMAs, since these are usually not exceeded by civil a/c and allow a rather quick stop of climb and descent without stressing the aircraft structure.

In cases where crews misunderstand ATC clearances / have finger trouble, they usually find out about the level bust either by ATC call, hopefully giving an immediate clearance, where to go or they encounter a TCAS TA/RA. I think that the "500 feet rule" would find only a very few situation where it could be used adequately without confusing the situation unnecessarily.

If we can be of any more help, please ask.


28. We support DFS´s proposed solution that an aircraft should change level to the next closest 500 ft interval but would like to raise the question about what to do when levelling off before assigned level. It seems wise to try to establish at least 500 feet separation to IFR-levels in those cases where the pilot is not able to directly establish contact with ATC. There might still be other IFR-traffic climbing or descending through the level but this s about trying to do the best out of a bad situation.

Another, maybe more infrequent, type of level bust is when an aircraft levels off before reaching the cleared level. For example, an aircraft cleared to flight level 330 levels off at flight level 310. If the crew recognises the mistake and are not able to directly contact ATC, should they start climb to the assigned level (330) or should they just climb another 500 feet to 315?


29. A very interesting subject indeed. I never thought of a level bust exceeding 500’ until now with the introduction of the fighter’s bust. We have trained our pilots to restrict the rate of climb/descent to 1000’/min for the last thousand feet before level off. Moreover, our pilots are trained to follow TCAS resolutions yet, I see no harm in issuing a recommendation in regard to the 500’ intermediate level should the 1000’ bust actually occur.


30. I'd like to make some points regarding TCAS equipage and VLJs.

As you know, under current regulations, VLJs are outside the ACAS II carriage requirement. In early 2008, we launched the ACAS on VLJs and LJs - Assessment of safety Level (AVAL) Project to assess whether VLJ operations, as currently forecast, will affect the performance of the ACAS safety net. The existing ACAS mandate was based on detailed safety analysis and the safety benefit to be accrued. Similarly, in this situation, decisions will need to be based upon objective (as far as possible) safety benefit data.

The Phase 1 of the project has concluded that there is likely to be an ACAS safety net performance implication. It has been recommended that AVAL Phase 2, which will seek to quantify the safety performance implications, should go ahead. AVAL Phase 2 (to commence in June 2008) will be a comprehensive safety analysis, to quantify ACAS II safety performance implications due to VLJs, both for the airspace and for an individual aircraft. The study is expected to be completed in mid-2009.

Secondly, one needs to remember that today not everybody has operational TCAS onboard. Several popular aircraft types, that fall outside the mandate and are not TCAS equipped, operate today in European upper airspace (including the RVSM airspace). For example: C551, C500, C501, C525, C25A, P180 - just to name a few. As you can see, if VLJs entered the European airspace today they would not be the only aircraft without TCAS II on board. Moreover, equipped aircraft can fly for up to 10 days with TCAS out of service under the MEL provisions.

On the issue of restricting the vertical rates - there is a change to Annex 6 pending (to become effective in November) that will limit the vertical rate in last 1000 feet. The exact wording of that change is not yet known (at least to me).


31. Introducing SIDs&STARs in terminal airspace with the level and speed restriction associated with excellent airspace planning will reduce level busting

The problem increased in RVSM airspace where 1000 feet separation is provided. 500feet as an emergency level is considered the best solution until having human recovery by the air traffic controller, in my point of view radar is an essential mitigation measure in RVSM airspace especially in congested areas (ICAO did not recommend obligation of using radar in RVSM airspace). Radar vectoring help in situation recovery.

STCA (as a safety net) must be adjusted for the air situation display for both RVSM and non RVSM airspace parameters at the same time (technical defence) and available for that dual purpose.

Annual refresher training which include aircraft performance(for all phases of the flight) for air traffic controller will increase personnel situation awareness leading to quick detection of level busting(training defence)

Level Bust Toolkit training for both pilot and controller.

Reducing the bidirectional routes to the most extent possible is an airspace design defence.


32. Congratulations with the initiative, I have however some reservations.

I am of the opinion that whatever new rule or procedure that will be invented or written in case of a level bust automatically brings along new and different potential risk. Then sooner or later something will go wrong.

Would it not be simpler to continue to stress the importance of listening better to the read-backs and correct, repeat and/or confirm level clearances; and for the pilots to cross-check the input of the cleared level input in their instruments.


33. Depending on the area where the level bust has occurred I would suggest a procedure on those areas with high density of traffic for the level bust incidents like the one that you are introducing as a possible solution.


34. Our feeling: this is not a good idea for the reasons several airlines have exposed.


35. I have read and concur with the comments as forwarded thus far. One however needs to distinguish between Commercial airliners and jet fighters wrt ROD / ROC and TCAS - I have flown both and am still on the B738. Once through the level (levels) it would seem dangerous to bust them again on the way back, especially for a fighter that has most probably busted quite a few. With commercial I do believe it might only be the one, but even so, if > than a 1000 feet I would agree to get to a “safe” level asap and to contact ATC. Here again and because we frequent the AFI air space, if not in contact with ATC then to use TCAS & IFBP if applicable. In all cases move with caution!

In view of the comments and the problem as given, I do believe a ‘briefing’ at least to be put forward just to keep safety awareness at a high. Airmanship to prevail and here one cannot cater for all scenario’s other than “if alt bust >1000 feet, get to the nearest 500’ level in between and communicate prior to trying to get back to the assigned level.”


36. This is a situation that requires further debate and consideration. The proposal for pilots to initially maintain 500ft from the cleared level is good provided that TCAS-RA where applicable is followed. The proposal should also be agreed upon by both pilot and ATC associations to avoid confusion.


37. We think that the suggested solution from DSF looks reasonable and the proposed 500ft emergency separation would help the situation until a new clearance can be issued.

A suggestion is to include this in Annex 2 and/or the operators manuals in addition to information to pilots (and ATC). Information about this should also be includet in Doc 4444.

In addition it is necessary with briefing/training for involved personell (aircrew and ATC). Necessary phraseology must also be developed.