Hacker News Reader

Comment by popeshoe | Hacker News Reader

I'm kind of astounded that modern planes haven't had this feature for a long time

It may be coming to larger military aircraft next.[1] Recovery strategies for fighters can be drastic. The F-16 auto-GCAS commands a roll rate up to 720°/sec, followed by a 5G pull-up. Only fighters and some aerobatic aircraft are capable of such aggressive maneuvers.

On the other hand, fighters are expected to fly fast and aggressively close to terrain. The goals of the F-16 auto-GCAS are

1. Do No Harm (don't initiate a maneuver that causes a crash)

2. Do Not Interfere (the pilot may be in an aggressive combat maneuver)

3. Avoid Ground Collisions

The conflict between 2) and 3) is tough. The rule of the F-16 system is not to interfere until a crash is less than 1.5 seconds away. This was established by flight testing with fighter pilots flying aggressive profiles that might be used in combat.

Larger aircraft are seldom flown that aggressively. Nor do they have the power and maneuverability to get out of trouble in 1.5 seconds. Today's GPWS and EGPWS systems provide up to 60 seconds of time from the warning to airplane impact. The FAA says "The GPWS mandate reduced CFIT (controlled flight into terrain) accidents from about 9 per year in the seven years immediately preceding the mandate to about 4 per year after. This rate has remained fairly constant". So there's room for improvement through automated recovery that isn't last-second.

[1] http://www.dtic.mil/dtic/tr/fulltext/u2/a618503.pdf

Look at all the benefits that come with FADEC (https://en.wikipedia.org/wiki/FADEC), and then ask me how widespread it is.

The aviation industry is veeeerry conservative, not to mention hesitant to retrofit perfectly functional airplanes.

It requires the aircraft to be aware of terrain around it. Most aircraft do not have topographical maps in their computers.

Not true, 95% of commercial aircraft have a TAWS system [0] that can show relative terrain on their navigation displays and provides FLTA alerts (forward looking terrain avoidance) by using an onboard terrain database. This is an FAA requirement for any aircraft with 6 or more passengers.

[0] https://en.wikipedia.org/wiki/Terrain_awareness_and_warning_...

I believe it work on a radar altimeter.

The terrain following radar feature that has been in fighter jets since the late 60s uses radar -- as the name implies, and serves a similar purpose. It allows the pilot to set an altitude and a "comfort" level regarding how aggressive the autopilot can be with regards to avoiding danger (basically, how quickly the plane can pull Gs to avoid terrain, and how many Gs it can pull).

That works well when you're straight and level, attentive, and the radar can point at the ground. This system can't rely on radar exclusively though because the aircraft may not have its radar pointing at the ground (as in the video, the aircraft is inverted in a pretty steep dive).

So, they have to factor in precision INS/GPS and known topology to assess terrain altitude in order to perform collision avoidance.

The F-16 system doesn't rely on radar, although it can use it, because fighters often fly with radar off. It tells the enemy you're coming. It's based on INS/GPS and a terrain database obtained from radar scans of the Earth made from the Space Shuttle in the 1990s.

Right, the Auto-GCAS feature doesn't rely on radar -- but the normal TFR system does, so the OP was half-correct in that there is a system that can use the radar to do ground collision avoidance... just not this particular system.

Dumb question, maybe, but how long before that data becomes inaccurate? Or rather, are there any areas where the change in elevation for the purpose of this system could be big enough in a 30ish year timescale that it would cause problems?

I assume no geological process alters the land drastically enough, quickly enough, that you'd notice, but what about water-level changes (dammed rivers?), melting glaciers, etc? Is "hard" ground consistent enough that no human processes are going to cause the data to diverge from the database drastically without the chance to update the database with new topographical surveys?

Reply on news.ycombinator.com