Skyguide claimed in a statement immediately
after the collision that it had received an unidentified TCAS descent call from
what it believed was the 757. The call's timing has not yet been clarified in
the investigation. Flight data recorders are still being analyzed at BFU's
Braunschweig facilities.
The 36,000-ft. altitude was correct for both
routes. Bashkirian Airlines Flight 2937 was a charter service from Moscow to
Barcelona heading almost due west, and had been under Skyguide control for about
5 min. It had been in German airspace where the route altitude was FL360, and
should have descended to FL350 for a different airway at the Trasadingen beacon
located a few miles beyond the crash point.
DHL's scheduled cargo Flight 611 was from
Bahrain via Bergamo to Brussels in a roughly northerly heading. According to
unconfirmed reports, the 757 had been handed over from Milan ATC to Zurich
(Skyguide) around 7min. earlier.
Skyguide's short-term conflict alert (STCA)
system was down for maintenance from around 11 p.m. local time. Zurich control's
main telephone line was also inoperable, and a second controller who normally
would have assisted the controller in charge had taken a break. During the
course of events, the controller was handling 4-5 aircraft on two frequencies
and two different screens. The controller tried to make a connection with
Friedrichshafen tower to coordinate an arrival between 11:25:43 and 11:33:11
p.m., but failed to get through on the reserve line, Skyguide stated.
MEANWHILE, GERMAN ATC provider Deutsche
Flugsicherung (DFS) confirmed that it received an STCA warning for the two
relevant flights at its Karlsruhe center. A controller tried to alert his Swiss
colleague in Zurich between 11:33:36 and 11:35:34 p.m. (a second after the
crash), but the telephone line was busy initially, and from 11:34:45 p.m. the
calls were not answered, DFS stated.
Peter Schlegel, head of the BFU, said that with
STCA down, minimum lateral separation for aircraft at the same altitude should
have been 7 naut. mi. instead of the regular 5 naut. mi. Skyguide should have
told the Tu-154 to descend to FL350 around 90 sec. before the crash to comply
with the extended lateral separation and with 1,000-ft. vertical separation, he
said. The first order to descend came 44 sec. before impact.
Meanwhile, Georg Fongern, a spokesman for German pilot union Vereinigung
Cockpit, said
the Tu-154 pilots should have
followed the TCAS command under any circumstances, neglecting ATC commands.
However, a Russian state official said that under Russian standard operating
procedures it was the pilot who would have ultimate responsibility for the
decision. The transponders on most domestic Russian aircraft don't work with
TCAS, and the domestic procedure is to follow ATC commands.
http://www.awstonline.com/cgi-bin/authenticate.pl?destination=docs/issues/20150207/aw36.htm
TCAS More 'Foolproof' Than Generally
Recognized
BRUCE D. NORDWALL/WASHINGTON
The traffic-alert and collision avoidance system
(TCAS), somewhat maligned and misunderstood in the wake of the
July 1 midair collision in Europe, provides more protection than is commonly
understood.
Pilots generally have a high regard for TCAS, and its European counterpart
ACAS (airborne collision avoidance system). From the
cockpit they are perceived as a safety net when the air traffic control system
fails to provide safe separation.
HOWEVER, CONTROLLERS have some reservations, largely because they have no way
of knowing what TCAS may be telling flight crews, and they are concerned about the results if there is a conflict between
the dictates of a human (controller) and machine (TCAS), as appears to have
happened in this accident. Reports of the mishap say that the DHL Boeing 757
pilots followed TCAS commands to descend, but the Russian pilots of the
Bashkirian Airlines Tupolev Tu-154 did not respond to ACAS commands to climb,
instead following the controller's order to descend.
TCAS will give protective warnings even from the approach
of a non-TCAS-equipped aircraft, or one whose TCAS is not working, so long as it
has an operating transponder. What is lost if both aircraft are not
equipped with TCAS or ACAS is the ability of the two systems to communicate
intentions.
A TCAS-equipped aircraft approaching within 20-48 sec. of another aircraft,
within 10,000 ft. of its altitude, will receive a traffic advisory (TA) with
both an aural and symbolic "Traffic, Traffic" alert, and a visual indication of
the vertical separation. If they continue to converge, the pilot of the TCAS
aircraft will get a resolution advisory (RA) directing him to descend or climb.
If both aircraft have TCAS, they will communicate to
avoid mirror-image maneuvers.
;) HONEYWELL |
|
The pilot's perspective in the white aircraft is shown
in sequence, from left to right in the top row, and then bottom row. A
stand-alone TCAS instrument is shown, but the display would be the same on
an Electronic Flight Information System, or glass cockpit. In the far left
display, (Non-Threat Traffic) RNG 10 is the range displayed
from own aircraft (blue cross) to the edge of the white circle. The other
non-threat aircraft (white diamond, open center) is 200 ft. higher (+02).
Proximity Traffic: The other aircraft is closer (white
diamond, white center) and still 200 ft. higher. A Traffic
Advisory is given when other aircraft (yellow circle) is within 48
sec. of closest point of approach (CPA), still 200 ft. higher.
Resolution Advisory--about 35 sec. before CPA--aircraft is a
red square, relative altitude (+200 ft.) is in red, and the vertical speed
indicator is green for the commanded descent (between 1,500 and 2,000
fpm.), with red showing that any climb or descent less than 1,500 fpm. is
wrong. Descent Established:The pilot has established a
descent with white needle in the green area, and vertical separation has
increased to 400 ft. Softening Command:With 600-ft.
separation showing in red, the pilot is directed to ease the descent rate
to between 0 and 500 fpm. down. Level Off: The white arrow
shows the aircraft is level, red number shows threat is 700 ft. above, and
aircraft symbol is still red. Clear of Conflict: As the
aircraft pass, with 700-ft. separation in the example, the target symbol
returns to yellow circle indicating proximity traffic. At this time the
pilots should notify ATC of the deviation and return to their assigned
altitude. |
TCAS commands avoidance maneuvers only in the vertical plane. When one aircraft's TCAS tells its pilots, "Descend,
Descend," it also directs the other TCAS aircraft to remain level or climb.
With the currently mandated systems (TCAS-II with version 7.0
software and ACAS-II), if a descending aircraft detects the
other also in a descent, it will make a dynamic adjustment and direct the pilots
to reverse the maneuver: "Climb, Climb."
THE PRECEDING TCAS-II, in use in the U.S. since 1994 employingversion 6.04
software, could make a dynamic reversal when a TCAS-equipped aircraft
encountered a non-TCAS aircraft that made a mirror maneuver. But it had a quirk
when such a conflict occurred between two TCAS aircraft. If they were both
descending to avoid contact, the algorithms could tell one to further increase
its descent rate, but could not tell it to reverse and climb. The7.0 software
introduced in 1999 and mandated in March 2001 has the capability to adjust,
according to Tom Staggs, formerly Honeywell's business manager for TCAS.
Honeywell is one of three manufacturers of TCAS-II, building the system
developed by AlliedSignal before it bought Honeywell, which spun off its TCAS-II
to be produced by L-3 Comm, now in partnership with Thales. The other producer
is Rockwell Collins.
A TCAS or ACAS installation has a radio transmitter and receiver, directional
antennas, computer and cockpit display. TCAS, like secondary surveillance radars
on the ground, sends out an interrogationto which transponders within range
respond. The TCAS computer uses the time between an interrogation and reply to
calculate the distance. Directional antennas give the bearing for the cockpit
display, but the collision algorithm is based on range rate--a constant change
in range (like a steady bearing) means that a collision will occur. The
transponder's Mode C or Mode S response gives altitude directly, which is
displayed as "+02" if the other aircraft is 200 ft. higher, or "-14" if 1,400
ft. below. Climbs or descents at rates greater than 500 fpm. are indicated by an
arrow pointing in the appropriate direction.
STILL, THE ISSUE of controllers not knowing what TCAS directions are being
given, and the possibility of conflict, is important. In the U.S., the directions are clear--obey TCAS.
Even the "general prudential" rule that instructions from air
traffic controllers are always to be obeyed unless an aircraft is in extremis
would indicate following TCAS. The logic would be that by the time a TCAS alert
is received, the ATC system has somehow failed to provide safe separation, and
so the aircraft is in extreme danger. TCAS
information on the other aircraft's proximity is updated one and perhaps two
times a second, compared with the controllers' information, which is updated at
the 12-sec. interval of a secondary surveillance radar antenna's rotation.
Staggs said trials to data-link TCAS resolution advisories to controllers
have been held in Boston. Europe is working to mandate delivery of TAs and RAs
to controllers over the Mode S transponder, but he points out that Mode S is an
inefficient modem by modern standards, with a data exchange rate of 300 baud, so
messages would have to be carefully designed.
