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| 5G versus commercial aircraft Radio Altimeters in the US, wtf? |
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| ve7xen:
--- Quote from: jpanhalt on January 19, 2022, 09:53:10 pm ---At first, I wondered why radio altimeters were such a big issue. You have both a barometric altimeter, which is usually set to the local pressure, and on approach you have at least glideslope. During the time I flew general aviation (US), two of my airplanes had radio altimeters, and I never used them except for amusement. ;) One problem is that when flying into airports in the SW and mountain states, the surrounding terrain can be well below the airport's elevation. That might be bad, if one descended to 200 feet AGL before reaching the threshold. Then I looked at the AD notices. Apparently, in modern airliners, the radio altimeter is linked to important landing functions, such as reverse thrust and automatic braking. The two "passengers" in the front seats cannot control those independently. Once obvious solution seems to be ignored. Why not have the pilots make those decisions? --- End quote --- I'm not a pilot, but I am pretty sure spoilers and autobrakes are typically triggered by the weight-on-wheels sensors, not the RA, and reverse is typically manual (though gated by weight-on-wheels), though there are probably some aircraft out there with different logic. These systems are armed manually or possibly by the aircraft during a managed approach (which might use RA to decide, but could easily use the INS altitude or some other signal), but activated when the aircraft touches down. If there's one lesson to take from the decades of aviation safety improvement, it's that humans are fallible, slow, and easily distracted, so reducing workload with automation is almost always a safety win. Having pilots do things manually would increase the amount of errors, both in the tasks themselves, as well as other errors caused by the increased workload during an already high workload phase of flight. The biggest user of RA is one that can't be replaced with something else - EGPWS, which is a safety system designed to save the plane when the pilots have lost situational awareness. Obviously, given its purpose, you can't replace this with pilots, and it depends pretty heavily on RA. It also provides altitude callouts (50-40-30-20-RETARD) which aid pilots in timing the flare, reducing human factors on landing performance, and the higher altitude ones provide procedural checkpoints. Also rather obviously essential for autoland. Of course you can get away without it, but you'll have sloppier landings and likely need increased minimum landing distances to account for that. |
| jpanhalt:
From the AD lint provided in POst #17: --- Quote ---The FAA determined anomalies on Boeing Model 787-8, 787-9, and 787-10 airplanes due to 5G C-Band interference which may affect multiple airplane systems using radio altimeter data, regardless of the approach type or weather. These anomalies may not be evident until the airplane is at low altitude during approach. Impacted systems include, but are not limited to: autopilot flight director system; autothrottle system; engines; thrust reversers; flight controls; flight instruments; traffic alert and collision avoidance system (TCAS); ground proximity warning system (GPWS); and configuration warnings. During landing, this interference could prevent proper transition from AIR to GROUND mode, which may have multiple effects. As a result, lack of thrust reverser and speedbrake deployment and increased idle thrust may occur; and brakes may be the only means to slow the airplane. Therefore, the presence of 5G C-Band interference can result in degraded deceleration performance, increased landing distance, and runway excursion. --- End quote --- |
| Halcyon:
Some reading released by the Australia Civil Aviation Safety Authority: https://www.casa.gov.au/search-centre/airworthiness-bulletins?search_api_fulltext=5G&field_dt_effective%5Bmin%5D=&field_dt_effective%5Bmax%5D=&sort_by=title&sort_order=ASC https://www.casa.gov.au/no-sign-5g-interference-australia Essentially, they have said: While CASA and the Australian Transport Safety Bureau (ATSB) have urged pilots to report any anomalies with radio altimeters near 5G towers, they have yet to see any. In fact, the ATSB says there have been no reports of radio altimeter incidents linked to 5G since the telecommunications technology rolled out 2 years ago. One reason for this is that Australian 5G transmissions currently do not extend into the part of the spectrum worrying the U.S. aviation industry. Radio altimeters operate in 4.2-4.4Ghz range and the 5G transmissions subject to the interference debate are in the adjacent 3.7-4.2GHz spectrum. Australian 5G transmissions currently top out at 3.7GHz, well below the radio altimeter frequencies. |
| LaserSteve:
It gets more interesting. To meet some modern reliability standards, on some aircraft the decision logic needs concurrence of. 2x WOW and 3x radar altimeter. It seems WOW has a lower reliability then radalt. Steve |
| ve7xen:
--- Quote ---From the AD lint provided in POst #17: --- End quote --- I stand corrected, though I really wish they would go into more detail on the logic. Doesn't really change the conclusion that you can't replace RA with pilots, though. --- Quote from: LaserSteve on January 20, 2022, 12:33:16 am ---It gets more interesting. To meet some modern reliability standards, on some aircraft the decision logic needs concurrence of. 2x WOW and 3x radar altimeter. It seems WOW has a lower reliability then radalt. Steve --- End quote --- It's hard to find authoritative information on this stuff, since the FCOMs and repair manuals aren't typically publicly available. Not too surprising that RA is more reliable than WOW, because moving parts suck. I'm really surprised that safety inhibits like reverser lockout would not 'fail open', and in fact that they make them even more likely to fail inhibit by adding more tests. I guess that's what the safety analysis concluded was correct, but it's surprising to me. Touching down and not being able to use spoilers or reversers is a pretty serious problem, but I suppose you just balk. |
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