Investigation Report May 2004
Kind of occurrence: Accident
Date: 1 July 2002
Location: (near) Ueberlingen/Lake of Constance/Germany
Type of aircraft: Transport Aircraft
Manufacturer/Model:
1. Boeing B757-200
2. Tupolev TU154M
Injuries to persons: 71 fatals
Damage to aircraft: Both aircraft destroyed
http://www.bfu-web.de/berichte/02_ax001efr.pdf
With the greatest of respect to the victims of the tragic accident at Ueberlingen on the 1st of July 2002, we offer the following extracts as an abject lesson in the folly of relying on TCAS, or see-and-avoid – as air traffic management design tools. See-and-avoid is ineffective for high energy aircraft, and TCAS is a last line of defence. The following extracts are from the accident report, available at the web-site noted above. They are made by investigators from one of the most respected accident investigative bodies in Europe.
We counsel anyone engaged in airspace – either as a pilot, controller, maintenance personnel, training staff or management – to read this sobering report. 71 people died as a result of the failure of the ATC system.
We counsel especially the architects of your NAS proposal to read and comprehend the consequences of flawed beliefs and political expediency.
Extracts from the final report:
“……Fundamental Purpose of ACAS/TCAS
ACAS/TCAS was developed as a collision protection system to be rated as the “last line of defence“. Similar to the stall warning system, the stick pusher and the ground proximity warning system (GPWS), ACAS/TCAS is to interrupt a possible chain of occurrences resulting from human errors or technical malfunctions and which may lead to an accident.
As TCAS II, Version 7 is an airborne device and issues resolution advisories to the crew only as visual and aural commands, obligatory procedural instructions for the utilization of and the reaction to TCAS advisories are indispensable. The TA alerts the crew in case of a potential conflict situation and requests their attention for a possibly succeeding RA.
The RA has the highest priority, because it will only be issued if other collision avoidance mechanisms, such as vertical separation by a controller, are not sufficiently effective or are incorrect. The manual intervention in the control of the airplane by the pilot must then take place without delay.
A coordination with the controller or a clarification of the situation by means of other airborne devices following an RA would question the purpose of TCAS. The time left in such a case could be too short for an avoidance manoeuvre and would increase the collision risk. According to general conviction, TCAS only makes sense if worldwide all crews rely on the system and comply with the advisories.
Thus it is the opinion of the BFU that with the system conception of TCAS II, Version 7 only one procedure can be permitted in case of an RA. The crew must comply with the RA without delay and report the initiated TCAS manoeuvre to the controller. Any other procedure which does not sufficiently take into account the priority of an RA would be contradictory to the purpose of TCAS……”
“…..Legal basis, procedures and procedural instructions
As TCAS II, Version 7 is designed as a semiautomatic system which shall serve as a “last line of defence“ in collision avoidance, clear and unambiguous procedural instructions for the crews are an essential prerequisite. This prerequisite is so important, because the system philosophy of TCAS II, Version 7 provides only one procedure after the issuance of an RA and that is to follow the generated RA.
The decision to follow an RA without reservation could mean that up to the resolution of the conflict the crew has to divert from other obligatory standards for instance, from instructions for vertical separation issued by ATC and from other general right-of-way rules……”
“…..TCAS 2000/TCAS II Traffic Collision and Avoidance System Pilots Guide
The specifications of the TCAS manufacturer's „Pilots Guide“ regarding the TCAS system philosophy and the necessary procedures which ensure a safe function were not described distinct enough. The wording „TCAS 2000 is a backup to the ATC (Air Traffic Control) system and the „see and avoid“ concept.“ could be interpreted that ATC takes priority to TCAS and that TCAS is designated to be implemental or a substitute. It was not made clear in the description of the system philosophy that TCAS is exclusively meant as a „last line of defence“ for the avoidance of a collision and that in this stage TCAS advisories must be disconnected from instructions given by ATC controllers.
„TCAS 2000 Pilots Guide“ does not state clearly enough that the safe separation accomplished through ATC and the tasks of TCAS are two different functions. It is not clear that TCAS is not part of the conceptual design of ATC……”
“……Defences
ACAS/TCAS
As an independent, onboard collision avoidance system, TCAS is designed as a last resort system to assist flight crews in avoiding a mid-air collision. The B757-200 VSI/TRA used for displaying TCAS information had a maximum display capability of 16 NM and holds a prevalent position within the pilots’ scan. TCAS is an onboard system which normally works in the background and becomes active once a collision risk appears.
The interface "TCAS - crew" becomes active with the generation of a TA. In this phase TCAS contributes to the situation awareness of the crew. An input into the control of the aircraft is not intended in this phase. After the generation of an RA the flight crew must take over control of the aircraft. Thereby is essential:
• Even in consideration of the pilot's final responsibility the TCAS RA must be followed. In this situation the crew has no better basis for a decision.
• Deviations from ATC instructions must be reported as soon as possible…..”
“…..Visual acquisition of aircraft
There is much literature about the physiological limitations of the human visual system within the aviation environment and particularly in the context of the “see and avoid” principle, often dealing with the difficulty in acquiring visual contact with other aircraft. This event exposes the human weakness even further, in that even with a visual contact with the opposing aircraft, neither crew successfully utilised the sensory information to recognise the high risk of a collision in time to successfully take avoiding action.
Having detected another aircraft a process of evaluation starts to assess the likelihood of a collision and to consider the need for evasive action. The first step of this evaluation requires that the relative position and relative motion be determined in all dimensions and then the rate of change evaluated.
At night and high altitude humans are extremely unsuited to this task.
At high altitude, and particularly at night, relative height it is almost impossible to judge visually with certainty, but change in the relative vertical bearing gives an indication of potentially conflicting traffic crossing above or below.
The lack of vertical change indicates no height difference at crossing, but does not allow determination as to whether a contact is above or below one’s own aircraft.
With high closure rates it may remain impossible to assess with certainty the relative height of intruder traffic until just a few seconds prior to the closest point of approach. Change of relative (horizontal) bearing is an indication as to whether a contact is passing ahead or behind one’s own aircraft, but in most environments the task of determining the intruder traffic’s heading still remains extremely difficult. In a potential collision situation the bearing does not change.
The third dimension involves the distance between the aircraft, and is assessed based on the apparent size of the contact. The relationship between distance and size is not a linear one, however: size increases exponentially with decreasing distance. At night and high level it is almost impossible to judge the distance between two aircraft based on visual information alone as there is no optical reference.
This makes it impossible to assess the closure rate until a contact is at a range that a change in size becomes perceivable. With a low closure rate the point at which a pilot can discern the size of another aircraft, and recognise the separation distance, may allow time to assess the flight trajectory and react to the situation prior to the closest point of approach. However, at high closure rates, by the time a pilot can detect a change in the apparent size of another aircraft, it will already be rapidly expanding in his view.
And although he instinctively recognises the high closure rate, even an immediate control input may not give enough time to effectively initiate an avoidance manoeuvre. The visual sensation created by a clear change of relative bearing, vertically or horizontally, conveys a sense that a collision threat is reducing, or does not exist. This perception is strengthened as the rate of change is increased. However, if the rate of change of relative bearing is reduced or is constant the pilot does not sense an increased threat of collision.
In this situation the pilot experiences an alarming visual sensation only if distance to the conflict traffic decreases significantly and the object size increases rapidly. The flight paths of the accident flights had the two aircraft at an almost constant relative height and horizontal bearing, but with a closure rate of about 710 knots (about 365 m/s).
This closure rate would not have allowed the crew of either aircraft to recognise the shortness of distance between them or their relative velocity till only several seconds before the collision. The remaining time was insufficient to decide on a course of action and affect a change in the aircraft flight path…..”.
“…..With ACAS/TCAS an additional safety system was introduced into aviation. It works independently of ground equipment and is installed in airplanes. ACAS/TCAS is a system of last resort and works independently of ATC units. Collision avoidance is one of the common tasks of the two systems. The instructions of both systems may command opposite directions. Yet, in case of an RA ACAS/TCAS takes priority, there is no contradiction….”