# Why were DHL611 and Bashkirian 2937 on a collision course?

This is about the crash over Lake Constanz/Überlingen on July 1, 2002. The planes were on a collision course, the automated TCAS told one to climb and the other to descend, but air traffic control gave the opposite order to the Russian plane and they crashed.

My question is, why were they on a collision course? I have not seen anything indicating they were off course. Don't planes flying different directions fly at different altitudes? And aren't there paths in the sky that planes are supposed to stay in?

What about flight plans? Does anyone check to see if two planes are on a collision course?

• I wouldn't say that They were on a collision course before then. Apr 13 '16 at 0:03
• @SMSvonderTann then why was a TCAS RA issued?
– fooot
Apr 13 '16 at 0:20
• Did you see the Deadly Crossroads episode of the Mayday series also known as Air Crash Investigation? I believe the problem was congested airspace around an airport. Apr 13 '16 at 0:45
• @fooot I know that, but it is hard to say that they were exactly on a collision course where they were originally. Apr 13 '16 at 0:52
• They weren't on a collision course until they were too close. The 2 planes were at the same FL but the controller had problems with the screens and telephones and couldn't advise them. He had also a lot traffic to control and he was alone. The pilots heared the TCAS warning but followed the wrong instructions of ATC and crashed Apr 13 '16 at 1:13

The Airways that the planes fly on criss-cross the sky and there are many, many intersections where there can be a potential for 2 aircraft to meet.

So yes, it is normal that aircraft are routinely flight planned to be on "collision courses". The flight plans don't keep aircraft separated. They are used so that Air Traffic Controllers can know the exact routing and plan where any conflicts may occur. It is normal and safe as ATC can then keep the aircraft separated using radar.

Generally aircraft flying eastbound fly at odd altitudes and westbound aircraft fly at even altitudes. This works quite well but we can still have 2 westbound aircraft both flying at 36,000' on a "collision course" because westbound can mean any track from 180 degrees to 359 degrees.

In this case the 2 aircraft were crossing paths at about 90 degrees to each other at the same altitude.

Normally they would be constantly monitored and the ATC controller would simply have one aircraft climb or descend to avoid the other aircraft. Another common practice is to tell one aircraft to turn slightly off course to avoid the other aircraft.

On this day the ATC system and the controller were both overloaded and a number of factors contributed to the controller not seeing the conflict until it was almost too late. At the last minute he told one aircraft to descend to avoid the other. This action alone would have been fine but the order was given far too late.

The Traffic Collision Avoidance System on the aircraft also saw the conflict and automatically issued an instruction for one pilot to climb while instructing the other aircraft to descend.

The controller did not know that the TCAS had also spotted the conflict and his command to descend was contrary to the TCAS climb command. The pilot chose to follow the controller's instruction instead of following the TCAS climb instruction. The other aircraft was also following his TCAS instruction to descend so both aircraft were now descending while still on a conflicting flight path routing.

TCAS is there as a safeguard when the ATC system fails. It should always take priority over an ATC instruction. It is clear that the ATC system failed these pilots and TCAS operated as it should.

As pilots we spend so much time following ATC instructions that I can see how it would be difficult to ignore the controller and instead follow the TCAS instruction.

• The rule that TCAS should always take priority over an ATC instruction is one of the safety recommendations after that accident. Apr 13 '16 at 8:46
• Based on an extensive analysis of TCAS II. Version 7.0 performance since 2000 performed primarily in Europe additional changes to improve the RA logic were identified. In response to a near mid air that occurred in Japan in 2001 and Mid Air that occurred in Ueberlingen, Germany near the Swiss border in July 2002, a change was made to permit additional sense reversal RAs in order to address these type of situations. If one pilot is not following the RA, then TCAS Ver 7.1 will issue a corrective RA. Apr 13 '16 at 15:22
• Thanks. I'm surprised that there isn't more of a system to prevent crashes, or alternatively that there aren't more crashes. I suppose planes are far enough apart that the chances of an actual collision are low even if their flight paths cross. May 4 '16 at 21:03
• @ttulinsky Why do we need "more of a system to prevent crashes"? We already have two systems (ATC and TCAS) and, as a result, there are almost exactly zero mid-air collisions, despite there being millions of flights a year. Aug 24 '16 at 7:08

Aircraft flying in different directions fly at same altitudes (FLs) at different times. If two aircraft were to to be found in the same FL at the same time, the ATC would assign different FL to atleast one of them, so as to minimize the risk.

In this case, the controller didn't notice that the two aircraft were in the same FL due to various reasons (mainly, he was concentrating on a delayed A320). From the accident investigation report:

When the TU154M crew contacted Zurich at 21:30:11 hrs for the first time and also reported the flight level, the controller did not notice that the B757-200 had reached the same flight level and that both airplanes were approaching each other at right angles. The distance between the two aircraft was still approximately 64NM. If he had noticed the situtation, he would have instructed the TU154M crew to descend to FL350.

The investigation listed this among the immediate causes of the accident:

The following immediate causes have been identified:

• The imminent seperation infringement was not noticed by ATC in time. The instruction for the TU154M to descend was given at a time when the prescribed seperation to the B757-200 could not be ensured anymore.