For years we have been able to rely on EGPWS as a critical barrier between us and terrain, and the reduction in CFIT events has been huge! But the impact of GPS interference is interfering with this as well. Here’s a look at how, why and what we can do about it.

What is the difference between GPWS and EGPWS?

First up, a mini recap on what they actually are.

They are both systems which help stop pilots flying into things. Things like the ground, or big mountains. You might have also heard them both under the general term ‘TAWS’ – terrain awareness and warning system.

GPWS, the older of the two, stands for ‘Ground Proximity Warning System’, and it uses radio altimeter data to provide warnings to crew when they get too close the ground and shouldn’t be, or get too close to the ground too quickly. In fact, that is really what they both do.: they yell at pilots when they’re about to get things very wrong.

It’s first form (GPWS) was developed and brought into commercial aircraft in the 1970s after a spate of CFIT (controlled flight into terrain) events, providing a critical barrier between pilots and terrain (in the form of a loud “Pull Up!” or “Terrain Ahead!” type order).

Great stuff… except, it has a blind spot. GPWS uses data that comes from the radio altimeter system, and this only really looks below the aircraft. So, if you have sudden steeply rising terrain, it isn’t going to spot it until the terrain has already had the chance to, well, rise steeply into you.

Which is why an ‘E’ was added.

E for Enhanced

The E of EGPWS is all about its predictive ability, made possible with its worldwide digital terrain database and the use of GPS. This means it not only knows where the airplane is, but knows what’s under it, around it and what might be steeply rising ahead of it, amongst other things.

Knowing where it is and what’s around means the system can “look ahead” and predict the flight path and any conflict it might have with the ground/hard things that will win that particular battle. The system will still provide reactive warnings, but it also gives predictive cautions and warnings, and these alert the crew earlier to impending issues.

When it first made it into the commercial industry (back in 1997) AOPA said that:

EGPWS is to terrain warning what the jacuzzi is to bath time – a true quantum leap in design.

But EGPWS has a weakness too, and one that we’ve only been seeing fairly recently.

Interference!

Specifically, GPS interference.

GPS jamming and spoofing mess with the system that the aircraft (and EGPWS) use to know where it is.

The impact?

Well, if it loses GPS then it no longer knows where the airplane is with as much accuracy so can’t predict much. All you’re left with is your basic reactive functions (which can give nuisance warnings). If it receives a spoofed position then it can be even more troublesome. Imagine you are up at 30,000’ and start descending, but your aircraft thinks you’re at 5000’ and heading towards terrain. It is going to yell some pretty aggressive things at you which you really won’t be expecting.

The more concerning threat is that you get no warnings when you should though. An aircraft that has lost its primary (and most accurate) navigation capability, heading somewhere with marginal weather, basic nav aid based approaches, stressed pilots and some challenging terrain is vastly more at risk of a CFIT event.

So the impact of GPS interference, which is becoming more and more prevalent, goes far beyond just ‘messing’ with the navigation system. It is removing a primary safety barrier, and one which we tend to have a big reliance on because up until now it has served us incredibly well.

How well? Since EGPWS starting becoming a more familiar feature onboard, this well!

The threat keeps growing

GPS spoofing is the nastier of the two interferences because, while jamming just tends to do what the name suggests, which basically ‘stops the system working‘ (technical description there), spoofing doesn’t break it but instead sends erroneous data into it. This is not easily identifiable – by the system or by pilots.

And, really, this is the biggest of the threats because it means pilots no longer no what to trust and what not to. Combine that with a reliance on a system that has been generally very reliable for the last 25+ years and we have a bigger problem than we might have first considered.

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GPWS to the rescue (sort of)

So, as we said, GPWS works off the radio altimeter and this is not affected by GPS interference because it doesn’t use GPS. So our basic, reactive warnings are still reliable.

But, and this is a BIG (and multiple) but:

• How do we know where the warning is coming from?

In the heat of a “TERRAIN AHEAD, PULL UP!” warning, are you really going to be able to weigh up the answer to this question quick enough? Probably not. And remember, this isn’t the only issue:

• GPWS can give nuisance warnings if flying into an airport with challenging terrain (one of the things EGPWS helped solve for us)
• If EGPWS is not inhibited, it can inhibit some of the GPWS reactive warnings (see point above)
• When GPWS is working, it still only really looks below, so your warning time can be down to 15 seconds, even less if the terrain is rising rapidly (the old limitation of the old system comes back with EGPWS out of service)

So, for all the “it still works” thumbs up for GPWS, it leaves us with a lot still to think about.

How should we think about it?

The industry is working on the GPS problem, but for now there are not so much fixes as ‘semi work arounds’, and this is very dependant on the aircraft you fly, your operator’s guidance and other official things. But overall, the main mitigation guidance for pilots seems to be:

Awareness!

And that is the entire purpose of this little post – to share some awareness of the issues the GPS problem means for our CFIT barrier, and what the industry* is currently saying we should do about it, and a recap on what methods we have available to us.

*By ‘The Industry’ I mean clever folk working in safety, at the manufacturers, operators etc who are working on this problem.

Awareness means not having to deliberate on it at the time!

In summary, the best guidance we are currently getting seems to be:

• Be aware of the impact on systems, such as EGPWS, and the additional threats this might lead to (see below)
• Understand the limitations of the systems we still have available and don’t be complacent or overly reliant on them (see below again)
• Monitor systems to identify early if they have issues, and be aware that degradation or erroneous information might be harder to spot than an all out failure (and also…)
• Be extra vigilant about monitoring our position, particularly in relation to obstacles and terrain (don’t get yourself into a position where you need TAWS!)
• Know when operating to an airport that may give nuisance warnings (so you can…)
• Brief the plan! What will you do if…/ what’s your Plan B/ and what about if…?
• Always follow operator/manufacturer guidance. Always! But that doesn’t mean not upping your own awareness and understanding of the problem so you understand that guidance in context
• Know your memory actions in case of a caution or warning. Reacting quickly and correctly is key
• If in doubt, always follow a warning! Don’t try to be clever, just get above the MSA and then work it out. It’s better to be embarrassed than be in the side of a mountain!