Level Busts -
Considerations for Controllers and Pilots
by P Tring and D Coxon, Principal Inspectors of
Air Traffic Services
In these days of high traffic volumes,
maintaining the intense vigilance required to guard against level
busts can be very stressful for the controllers involved. It is,
however, important to maintain a sense of perspective. During 1996
there were 172 level busts reported in the UK - a small number given
the millions of flights within UK airspace. There is no room for
complacency though. The overall trend is a gradual rise in level bust
incidents as traffic increases but, even if we maintain the same
percentage of level busts per flights operated, the increasing number
of incidents may be unacceptable. Therefore, we must reduce the number
of level busts. There is also no room for finger pointing. Level busts
can stem from any number of causes and in all phases of flight. It is
not for pilots or controllers to blame each other; instead they should
work together to reduce and, if possible, eliminate level busts.
This article aims to give an insight into some
of the techniques being adopted by controllers as “best practice” to
try and reduce level busts. As a pilot reading this you will hopefully
understand why controllers operate in particular ways, and therefore
increase your own awareness of the situation. As a controller meeting
these ideas for the first time you will perhaps discover some
techniques to enhance your controlling. Some of them may seem
blindingly obvious but it does no harm to remind ourselves of what
good technique is: sadly, all too often it is found to be lacking when
incidents are investigated.
Firstly, communication! Since controllers and
pilots communicate verbally, it is vital both parties understand what
is being said. Sometimes, for perhaps one or both parties, English is
NOT the first language. If we do both speak English then avoid going
"native" and using colloquialisms that are often more likely to be
misunderstood, and do nothing to help third parties using the same
frequency maintain some degree of situational awareness. So let us
ALL use clear unambiguous STANDARD PHRASEOLOGY.
If somebody is having trouble understanding,
then treat them with more care. Be alert for strangers to your airport
or airspace, alert colleagues and adjacent units to the potential
problem. If you work at an airfield why not try and make contact with
new operators, perhaps via the handling agent, to describe ATC
procedures in use and highlight possible difficulties? As a pilot why
not visit air traffic to discuss your operations and learn how the
local airspace is managed?
Continuing on the theme of communication,
controllers should avoid multiple messages and be aware of the pilots’
circumstances. Imagine yourself with a B747 strapped to your posterior
having been up all night, perhaps having crossed three times zones
before staring into a blinding sunrise and, to cap it all, a
controller tells you to descend to this level, by this DME distance,
fly that heading and this speed and by the way there will be a forty
minute hold for landing! There are just too many numbers, just too
much opportunity for confusion, it is just too easy get figures mixed
up and have an aircraft descend to your heading! Split instructions
into two or more transmissions with instructions paired unambiguously
and, having heard a correct readback, allow the pilots enough time to
implement those instructions before passing more.
"Traffic information", or is it "information on
traffic"? Experience has shown that reference to the level of
conflicting traffic, often given in response to a request from an
aircraft for further climb or descent clearance, can be mis-interpreted
and result in the aircraft continuing to the level of the conflicting
traffic. Many controllers now often respond to requests for further
climb or descent with, "Roger, maintain flight level, traffic
................., a thousand above or, a thousand below", as
appropriate. This information is not strictly, "traffic information",
as described in the Manual of Air Traffic Services, but "information
on traffic". There are plans to amend the manual so that this
difference is recognised and reflected in standard phraseology.
Remaining with the theme of requests for further
climb or descent clearance, the only response to such a request should
be "Roger, maintain flight level/ altitude (actual cleared level),
(with information on traffic if appropriate) or a further climb or
descent clearance. NOT some slick sounding reply like, “Coming soon,
further in 5 miles inbound company traffic crossing etc., etc."! Such
verbosity uses up valuable R/T time and can create further
opportunities for mis-understanding.
Controllers within LACC Area Control, and
elsewhere, often find themselves, having given initial climb
clearance, asking pilots for their requested cruising level. The
opportunities for mis-interpreting what is a question and what is a
clearance are easy to appreciate. Good practice is that following the
question the controller should acknowledge the level requested and
then RESTATE the current cleared level. Example,
Controller - "Big Jet 101 climb FL 280, what is
your requested cruising level"
Pilot - "Roger, Big Jet 101 cleared FL 280 and
we are requesting flight level 350"
Controller - "Roger Big Jet 101, maintain FL 280
on reaching"
It may sound really obvious, but you might be
surprised how often the result of similar exchanges has been the
aircraft continuing climb to the requested level, sometimes even with
English speaking crews!
Some controllers often omit repeating the level
when it is given by a pilot establishing first contact. The potential
for an incident if the controller should mis-hear or make a mistake
when acknowledging the level was illustrated in the Aircraft Proximity
Reports Airprox (C) Vol. 12, report 24/96. Some pilots from states
where a non-standard form of ICAO phraseology is used can interpret
the expression, for example "maintain FL310" as clearance to climb or
descend to that level if not already maintaining it.
Pilots may have noticed that controllers often
now come back and confirm the cleared level and give "information on
traffic" as the cleared level is approached. This has developed in
response to the activation of the Short Term Conflict Alert (STCA)
available on some radar displays. The controller will, if workload
allows, reiterate the cleared level if the radar indicates that no
horizontal separation will exist between aircraft, i.e. only vertical
separation, a situation where failure to maintain the cleared level
could become immediately critical.
The opportunity for level and heading
instructions to become transposed has long been appreciated. A
technique being used to help reduce this is to try and use, if the
tactical and airspace situation allows, headings that end in
increments of “5” degrees. Another is to use the word “degrees” at the
end of a heading instruction. These are most effective in airspace
where headings and levels are in a similar banding. There are sectors,
for example, where the most commonly used levels are FLs 100, 110 and
120 and the most common headings are from 090 degrees to 120 degrees.
The potential for confusion is obvious!
There have been cases of aircraft, after flying
a standard instrument departure (SID), climbing above the highest SID
altitude directly to their requested cruise level without ATC
clearance. The aim is to change the UK Air Pilot (AIP) requiring
pilots to state their cleared level or altitude on first contact with
departure radar. If this information is not given then the controller
is responsible for confirming the level. In the case of those UK
airports with "stepped" SIDs, then the controller must confirm that
the aircraft is/or will follow the correct profile. Due to an embargo
at present on changes to the UK AIP, because of a major re-write, this
change will initially appear as an Aeronautical Information Circular (AIC).
If as a pilot or controller you should be
unlucky and be involved in a level bust incident what should you do?
Firstly, whether you are a pilot or controller, make the situation
safe, then make other parties involved aware that reporting action
will be taken even if only one aircraft is involved. Do not become
embroiled in discussion and argument over responsibility on the
frequency! Such un-professional action is a distraction from primary
tasks and could lead to another unrelated incident. Safeguard the
evidence for those reports and then get back to concentrating on your
job. If you feel discussion would be of benefit then arrange quickly
to make contact later. R/T and radar recordings will normally provide
all the information required to investigate the incident. We in the
Safety Regulation Group rely on your full, prompt and honest reports
to identify causal factors and thus improve air safety for everyone’s
sake.
We hope the preceding has been of interest to
you and that it will set you thinking about your own technique whether
pilot or controller. The best way to beat level busts is to increase
our awareness of the problem, educate ourselves about best technique,
and then maintain our professional discipline to make sure we always
do things the best way. If this all encourages you do that then it has
been worthwhile, and if it stops even one level bust it will have been
even more worthwhile, because that could be a tragedy averted.
Finally, for pilots, if you have any doubt
whatsoever about an ATC clearance then seek confirmation. That does
not mean repeating the clearance in a quizzical, questioning tone.
Such subtle intonation cannot be guaranteed to be recognised by
controllers, especially if they are busy. The sure method is to
clearly state, "CONFIRM that was for (callsign)", or, "SAY
AGAIN our cleared (flight level/ altitude/height)". For
controllers, listen to those readbacks! If in doubt, RESTATE
the clearance and ASK for clear confirmation. Do not use
readback time for getting on with the next task., Remember, failure to
spot an incorrect readback was cited as the biggest single cause of
controller derived level busts during 1994!
GOOD PRACTICE TO AVOID LEVEL BUSTS
ALL
Increase vigilance, particularly in TMAs and
where traffic density may be high
If in doubt, confirm on the RT, not with your
colleague
PILOTS
Seek confirmation from ATC if in any doubt
about a clearance
Follow SOPs for vertical clearances in their
entirety - they are your first "defence" against busting a cleared
level.
CONTROLLERS
Handle unfamiliar operators with care.
Use clear and unambiguous standard phraseology
Avoid multiple instructions
Pay particular reference to readbacks
Awareness - contact operators via handling
agent/airline operations
Restate cleared level when asking requested
level
Restate cleared level - if no horizontal
separation
Avoid reference to level if giving
"information on traffic"
Consider using heading instructions that end
in "5"
Confirm cleared SID level or altitude at
departure
Controllers within LACC Area Control, and
elsewhere, often find themselves, having given initial climb
clearance, asking pilots for their requested cruising level. The
opportunities for mis-interpreting what is a question and what is a
clearance are easy to appreciate. Good practice is that following the
question the controller should acknowledge the level requested and
then RESTATE the current cleared level. Example,
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