If you look into how aircraft keep clear of obstacles while flying you’ll find yourself awash in acronyms. GPWS, TAWS, TCAS…..for pilots and air traffic controllers these are vital systems that have drastically improved flight safety in recent decades. They’ve helped to reduce incidences of “CFIT” – or Controlled Flight Into Terrain. And yes, it’s as undesirable as it sounds. As passengers it might be reassuring to know a little about these systems and how they work. So here’s a look at some of the technology behind how planes avoid running into things.

La Paz El Alto Bolivia terrain mountains GPWS warning safety systems how planes avoid running into things
Caution terrain. The airfield at El Alto, the airport serving Bolivia’s capital La Paz.


Terrain Awareness & Warning Systems

Let’s begin with TAWS, which stands for Terrain Awareness & Warning Systems. That’s a generic term to cover a handful of systems that provide automated aural warnings when an aircraft gets close to the ground. The original system developed in the 1970s was called GPWS, or Ground Proximity Warning System. It used the radio altimeter to determine whether an aircraft was close to terrain. Later enhancements were added so that it would warn about deviations from the glideslope or improperly configured landing gear and other unsafe states for landing.

Innsbruck Airport mountains
A night scene at Innsbruck-Kranebitten Airport in Austria.

In the late 1990s a so-called EGPWS (Enhanced Ground Proximity Warning System) was developed to improve things by incorporating GPS data from the aircraft and a global database of terrain. A modern TAWS system will keep a lookout for terrain ahead of the aircraft and warn pilots if anything is coming close. It will also warn about excessive descent rates and other potentially unsafe states. If you’ve ever been in a flight deck for landing you may have heard the automated call-out that says “five hundred” when the aircraft is 500 feet off the ground. The TAWS system is responsible for that as well.

Obviously the best way to avoid flying into terrain is to have a skilled pilot who knows what they’re doing. But all kinds of complications can arise, from low visibility to other safety issues causing momentary distraction and more. These systems are in place to step in under those conditions.

ATA Airlines Iran mountains terrain Tehran Iran MD-83
An ATA Airlines [Iran] MD-83 lands at Tehran. The city is surrounded by terrain that pilots need to be aware of.

Warnings ignored

These systems figured in the recent crash of Pakistan International Airlines flight PK8303 in Karachi (KHI). In that crash the pilots initially touched down on the runway with the landing gear retracted. That caused the engines to scrape the runway and become damaged. They initiated a go-around but went down shortly after. It was surprising that the pilots could manage to put the plane down on the runway without the landing gear down, because the A320 would have had an EGPWS system that sounds a clear warning to pilots should they attempt such a thing. Sure enough data and recordings from the crash indicate that those and other warnings were triggered. The pilots appear to have ignored them. Suffice to say these systems are very effective, but only if acted upon by the pilots.

Steering clear of other planes

In modern aviation we have highly effective air traffic control systems, assigned flight paths and tracks, and very capable controllers looking over the whole system to ensure every aircraft keeps a safe separation from others. But occasionally things happen and on very rare occasions two planes might find themselves on a literal collision course. Because of the speeds involved at cruise there would be little chance for pilots to react in time, and even if both did react there’s no guaranteeing they would do so in a coordinated way so as to move in opposite directions from each other.

Enter TCAS, or Traffic Alert and Collision Avoidance System. It is also sometimes referred to as ACAS (Airborne Collision Avoidance System). This system works independently of all other systems onboard, simply keeping a lookout for transponders of other aircraft in the vicinity. Taking into account the altitude, speed and current flight path of the aircraft it’s installed on plus any others that are nearby, it automatically keeps an eye on whether any other aircraft may represent a threat.

sunset contrails how planes avoid running into things TCAS traffic collision
An All Nippon Airways Dreamliner cruises overhead.

If it does perceive a threat, the system will issue one of two warnings to pilots on both aircraft involved. The first and less serious is a TA, or Traffic Advisory. That indicates that two aircraft are have come within a certain distance of each other and its aim is to make pilots aware of the situation. If the aircraft continue to get closer to each other, at a certain point closer in the system will trigger an RA, or Resolution Advisory. In this case the systems coordinate between aircraft to provide complementary suggestions for the two aircraft to establish a safe separation – in most cases telling one to climb and telling the other to descend.

It’s an elegant system in that its warnings to pilots are very simple even as the underlying tech and algorithms employed are quite complex. And it’s a very useful “last-resort” safety feature. For more on the details, see this text.

747 vertical separation TCAS airplane safety
A 747-400 seen passing 1000 feet overhead while in cruise.

Aviation has evolved steadily over the years in order to become safer and safer. Commercial aviation in particular has an impressive record of taking a hard look at mistakes and oversights and learning from them. And the systems that keep us clear of the ground and other aircraft have had a measurable impact on reducing the number of accidents. On most flights they won’t be needed for anything more than routine reminders, but it’s certainly good to know they’re there.

Featured image © Mason Wang