Terrain and Traffic Collision Avoidance

It’s almost licence skills test time and incredibly, I actually feel quite prepared for it. We’ve been given a consolidation full motion lesson tomorrow wit the next day being the test. It’s been the last few lessons where I feel that my knowledge and skills have come together to produce a pilot that actually feels in the mix with what is going on, is and has spare capacity to deal with other things. I just hope it stays that way during the test!

On Monday, amongst other things, we looked at terrain avoidance and traffic collision avoidance.

Terrain avoidance warnings are achieved by systems called (Enhanced) Ground Proximity Warning Systems. These systems use sensors on the aircraft to detect situations such as  excessive terrain closure, excessive descent, altitude loss after take off, unsafe terrain clearance and a few other things. They were invented after a aeroplane crashes involving controlled flight into terrain started becoming the most common cause of hull loss.  In the 1970’s when the first GPWS’s were installed on board aircraft however, they were restricted to only been able to sense what was directly under the aircraft, this meant that any dramatic change in terrain could cause the GPWS warning to come too late for the crew to react. A later version of the system came with a resolution to this problem, an on board database of the worlds terrain. This system known as the EGPWS is able to calculate, from an aircraft’s flight path and it’s terrain database, whether the flight path leads into terrain. It will then issue a warning saying something along the lines of Terrain Ahead and using the navigation display you can modify your flight path to avoid the terrain. A clever system to say the least and with the addition of being able to throw the side stick fully aft without any consideration for over stressing the airframe or stalling you are able to achieve a huge immediate rate of climb seconds before any aircraft without those protections. We flew out of Innsbruck and were told to fly towards a mountain, at the pull up terrain warning we were able to get about 9000feet per minute rate of climb out of it.

The different colours represent the different heights of the terrain, green, yellow and red, the highest being red and the lowest being green.

TCAS or Traffic Collision Avoidance System, does exactly what it says on the tin. There are various types of TCAS that give information from other aircraft positions to resolution advisories allowing you to climb or descend out of danger. The TCAS in the Airbus has a lot of functionality, it will tell you were intruders are, up to a range of 80nm’s ahead. The level of alert or notification you receive depends on the threat of the intruder, these are proximate’s (no threat), Traffic Advisory (Potential collision threat with approximately 40 seconds until the aircraft cross paths) and Resolution Advisory (A real collision threat  with approximately 25 seconds until aircraft cross paths). The resolution advisory can either be uncoordinated or coordinated, depending on the level of TCAS on board the other aircraft, however for the TCAS system to see another aircraft they have to have a transponder transmitting at least the aircraft position. For resolution advisories the other aircraft needs to be transmitting it’s altitude as well.

In the above image you can see the red square is a resolution advisory, the aircraft is 100 feet below and climbing. Over to the right there is a traffic advisory, it’s 40 seconds until both aircraft cross paths. It’s 1000 feet below and descending so probably not a problem. The white proximate, the filled in diamond, is 1100 feet below and climbing. It may or may not become an RA, depending on where it levels off and it’s flight path. The hollow white diamond is when the aircraft isn’t close enough for it to become a solid white diamond. When the aircraft gets within a certain range the diamond will become solid if it’s still deemed to not pose a threat.

The alerts for both TCAS and Terrain Avoidance, as you would expect, are very loud and have a lot of urgency. They have to be acted upon immediately to keep the aircraft safe and even in the sim you feel a tense feeling, hoping that you don’t the imaginary aircraft that you can see coming towards you on your navigation display or that piece of high ground you’re trying to climb over.

Anyway back to revising for my LST on Friday!


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