Airborne
Connections
Air Safety Forum 2
| Off-Set Tracking |
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| A case for Off Set Tracking |
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Of
Slammed Stable Doors and Bolted Horses
By Capt Chris Young Aviation accidents caused by one single, cataclysmic error are rare. The vast majority stem from a succession of small, apparently insignificant events, many of which would not even be classified as errors in isolation. However, when put together in an ‘event chain’, they provide a unique set of circumstances that allows one final ‘event’ to provide the link necessary to ensure what is often disaster. Most contemporary accident investigations will list some such event chain leading up to the accident and with the benefit of hindsight, the investigator will point out how easily the accident could have been avoided. If the crew (or ATC, or airline management, or a flight or ground training organisation, or the aircraft manufacturer or a regulatory authority) had broken the chain by using a simple (and now – with that wonderful 20/20 hindsight – perfectly obvious) commonsense procedure, all would have been well. With months to peruse each and every piece of evidence, (and to reach conclusions that the unfortunate crew sometimes had to arrive at within seconds), the investigator will make recommendations to ensure that an accident in similar circumstances is less likely to happen again. This is usually very constructive criticism, and there are few of us who have not amended our own work practices after reading some such report. It is seldom that anyone could have foreseen the need to break the (now so obvious) event chain detailed in such reports, for it goes without saying that if anyone believed that some procedure might be potentially dangerous, they would have informed someone in authority. Just as surely, that person or agency would have done something about it. Wouldn’t they? History says this isn’t always so. There are few people in positions of authority in today’s aviation industry who aren’t overworked to the point where the last thing they want is another problem dumped on their desk, especially if it is an issue that no one else but some malcontent perceives to be a problem. If a procedure has been accepted as safe practice for many years within the industry and some fool starts banging on your door insisting it is unsafe, he’s got to be wrong. Hasn’t he? Let’s investigate this event chain idea. Simply put, it says that all it takes to prevent an accident is for someone to break just one link in a chain of events. In an ideal world, this break in the chain might be achieved very early, perhaps within an airline’s initial training programme or even its selection process. At the other extreme, it could be the at the very last link in the chain before disaster strikes, perhaps a pilot taking frantic avoiding action as a mountain top looms out of the mist. Of course, the break might also occur anywhere in between. I think few of us would disagree with the notion that we’d prefer to see the ‘chain cutter’ applied successfully to a link as far from what I’ll call the ‘pointy end’ of the chain as possible. It is with that view in mind that I sat down to write on the subject of mid air collisions and avoiding them. A case for Offset Tracking The current air route system was designed in the days of VOR tracking and celestial navigation. Tracking tolerances were wide, narrowing down to any exactitude only approaching the overhead cone of an enroute navaid. Thus, even if an error in Flight Level allocation was made, the crews of two conflicting aircraft would have had to be extraordinarily unlucky to suffer a head on mid air collision. The chances of two aircraft being in exactly the same piece of airspace as they passed were very slim indeed. With the widespread introduction of IRS and GPS navigation systems, this is no longer so. The GPS navigation systems employed on the latest generation of airliners assures a constant tracking accuracy within 30 metres at all times. That’s well within the wingspan of any widebody airliner and most narrow bodies. Translated into plain English, that means that if two GPS-equipped aircraft flying reciprocal tracks should happen to find themselves at the same level, they will, without a shadow of a doubt, most assuredly collide if avoiding action is not taken by one or both crews. To add to the certainty of collision is the digital altimetry systems in modern aircraft. In the days of aneroid altimeters, opposing aircraft at the same Flight Level might have had anything up to 200 feet vertical separation at their standard cruising levels. With the new ADCs, they will be at exactly the same level. I’ve heard the argument that offsetting is simply not necessary because TCAS will ensure sufficient warning is given to both crews if this highly unlikely event was to occur. (It is, I agree, unlikely, but so is an engine failure right at V1, yet we faithfully practise that particular emergency twice a year in our licence renewals. Likewise, I’ve never done a genuine emergency descent in my airline career, yet I am made demonstrate it frequently in the simulator because someone thinks it might happen, and if it does, they believe I should get it right the one time it will really matter.) In answer to the TCAS argument, I go back to the chain of events theory. Why rely on the last link in the chain when at little or no cost, we can snip the chain far earlier, (and importantly, much further from the very self-interested 'pointy end' of the chain – you and me)? And that last link doesn’t always work. I have had personal experience within the last two weeks of passing a major flag carrier aircraft within the TCAS ‘window’ with no readout at all on my nav display. Within ten minutes, another aircraft from the same company passed and we got a normal readout. My suggested early snip of the chain is not to fly in exactly the same piece of vertical airspace. The analogy of a speeding bullet is compelling and given the closing speeds of jet aircraft in the cruise, quite accurate. If you knew someone was going to be firing a bullet along a fixed path every ten minutes with an assurance that he would be aiming above your head, would you walk down that line or to one side of it? Just in case. Another two dimensional example is to imagine you’ve been told to drive from point A to point B by the shortest possible route as fast as you can. To make life easy, there’s even a straight line drawn along the ground between A and B for you to follow. You are also told there’s a car coming in the opposite direction with the same instructions. If you could confer with the other driver before setting out, would you both plan to drive with your wheels straddling the line and take avoiding action when – and if – you see each other, or would you agree to drive to one side of the line? (Please, no "I’ ll take the left side an’ you take the right side" jokes.) The next argument is that there are many other checks and balances to ensure the last link of taking avoiding action will not be necessary. However, all these checks and balances rely upon men who are safely sitting on the ground in ATC control centres. They are always busy, of varying levels of proficiency, and their ATC organisations can often leave a lot to be desired. (Anyone who has flown over Africa will be unlikely to disagree with that last point.) If they make a mistake, in some countries they may have to face a court case for negligence. They may also suffer a terrible bout of conscience, which will be of little comfort to the hapless pilots and passengers who were the subject of their error, for they will be dead. We are, as airline pilots, are quite possibly the most conservative group in the world. By our very nature we are resistant to change, which is no bad thing. However, this can be taken too far, and in the interests of breaking a potential error chain that could have catastrophic consequences, I believe we need to demand change from our hard-worked supervisors on the subject of offset tracking. Some regulatory authorities acknowledge the problem exists, but they can be forgiven for hastening slowly, for any such changes will cost money, and besides, their customers, the airlines, are not beating down any doors demanding it be implemented. Like IFALPA, the Australian CASA is advocating a worldwide embedded (automatic) right offset of .1nm for operations above 10,000’. While it is a move in the right direction, I (and CASA’s own experts) don’t believe .1nm is enough lateral separation. At the very minimum, it should be .5nm and preferably 1nm. However, I am told that anything more than .1nm will involve a re-survey of the air route system, which would involve huge expense. Given the accuracy of the new navigation systems, I would propose that a far more cost-effective but still safe way around this problem would be to stick with the current surveys and reduce the paper safety tolerances on the air routes’ extremities for GPS / IRS-equipped aircraft. I cannot see this being a problem, for only aircraft equipped with very accurate GPS / IRS equipment would be employing the embedded offset, which would still leave them within the confines of the air route. Aircraft not so equipped would still fly down the centre of the airway. This proposal is not a panacea. It will not protect an aircraft from crossing traffic nor from aircraft using less accurate navigational equipment. (Some would argue that these older aircraft, less likely to be equipped with TCAS, are far more likely to be found without clearance at non-standard levels. The same people might say that by far the safest place in the sky to avoid these errant aircraft might be exactly on the middle of the airway!) There is no way to ensure 100% flight safety short of grounding every aircraft in the world, and even then, you could be sure somebody would regularly run into one on its parking stand. Collision with crossing traffic, whilst a concern, is statistically far less likely than with an aircraft on a reciprocal course. For crossing traffic to collide, both aircraft have to occupy exactly the same piece of airspace for that one fleeting moment as the two tracks intersect. However, with GPS, reciprocal traffic can be tracing exactly the same path for eight or more hours at closing speeds literally faster than a speeding bullet. The chances of collision go up exponentially, particularly if for some reason that last link in the chain, TCAS, malfunctions or is not fitted. If enough of us press for enroute offsetting, maybe just this once the stable door will be closed before the horse has bolted, and maybe on the way we can save up to 800 lives by preventing another disaster like the Saudia / Khazak Airways mid air near Delhi. The life you save might even be your own.
A second major regional carrier has been authorised to conduct a long term fleet-wide trial using one nautical mile offsets to the right of the planned track or ATS route by its local regulator. The authorisation is as follows: Offset Navigation Procedures Trial At the Captain's discretion, a 1.0 NM right offset may be flown in areas where there is considered to be a higher than normal potential for traffic conflict. This offset navigation procedure is subject to the following restrictions:
Crews are reminded that the IATA Technical Policy Manual already states that when flying in areas where In-Flight Broadcast Procedures (IFBP) arc used, e.g. Southern Africa routes, all aircraft should fly offset 1.0 NM right of the nominal track. Crews are requested to comment via the Voyage Report on the effectiveness of this procedure or any problems encountered, either with ATC or other aircraft. A summary of comments will periodically be sent to the Regulatory Auority. Background The introduction of FMS navigation equipment with IRS and GPS update capability has effectively eliminated the element of inaccuracy in aircraft navigation. Paradoxically, this improved navigation accuracy has, in fact, been detrimental to the overall safety of the air tragic management system because it has concentrated traffic on the exact centerline of the airways. Introduction of a 1.0 NM offset procedure should improve safety by reducing the concentration of traffic on the airway centrelines. Information gained from this trial will be correlated and passed to the Regulatory Authority to study and to share with other Regulatory Authorities.
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