United flight descends 28,000 feet in 8 minutes for ‘pressurization issue’ | Hacker News Reader

United flight descends 28,000 feet in 8 minutes for ‘pressurization issue’

cnn.com

21 pointsmjirv23 comments

Those numbers look scary at first sight, by that doesn't seem like a particularly fast rate of decline I think? IANAP, but a normal fight gets to a cruising altitude of ~35k feet in ~ 10 minutes from ground level I think? So this was not some meteoric plummet.

That being said, obviously there was some serious safety issue, so kudos to the pilot for quickly and safely securing the aircraft

Correct, the headline equates to 3,500ft/min. It's about double of the usual rate of decent, but still common enough that regular passenger carriers will sometimes decend at that rate.

Rule of thumb is that 1 kn ≈ 100 ft/min.

So 3500ft/min is about 35 knots vertical speed. Not much compared to the horizontal speed, which is closer to 500 kn.

40mph straight down is a pretty fast rate of descent. That said there’s nothing inherently unsafe about it. If you’re losing cabin pressure, you really want to be at a breathable altitude ASAP.

Yeah it's definitely steep, but just to put it in perspective it's something like a 7% gradient

I have been in several models of commercial aircraft during certification testing where we had to perform a max effort emergency descent as a part of the flight test profile. We routinely would hit rates of descent in the of 9,000-10,000 FPM range. I normally experienced this while strapped into a flight test workstation in the back of the plane, but once or twice had the opportunity to sit in the flight deck jumpseat.

Once the aircraft is established in the descent, there is a bit of additional buffeting (light turbulence) and wind noise, but there is no highly unusual attitude involved. Initially, depending on how aggressively the pilot transitions from level flight to the descent, there can be a brief second of reduced or even zero G, but a smooth transition does not increase the total descent time by too much.

I have a colleague who was involved with the flight testing of the C-17 military cargo plane. That aircraft can safely deploy all four thrust reversers while in-flight to make a high angle tactical descent - it can hit in excess of 25,000 FPM. Supposedly everyone on the test flight was supposed to have been medically cleared for this flight, but one crew member had a lingering ear infection, and blew out both eardrums during the test.

I did a Space-A flight with my family on a C-17. They didn't do the full thing, but they absolutely pushed that bird up and down hard, with some banking, on takeoff and landing. They warned us, and a good time was had by all.

Worst landing I ever had was a dirt field in a fully PAX loaded C-130. It wasn't the descent rate, it was just the landing. No thanks.

Track log on flight aware shows max decent rate around 6,000 ft/min.

Ascent to cruise was around 1,000-2,000 ft/min.

That's about 70mph vertically downwards.

According to https://aviation.stackexchange.com/questions/32111/what-is-t... it's a relatively safe speed in an emergency. Note, speed brakes are deployed to increase drag, which helps increase vertical descent without overspeeding the wings.

The plane doesn't need to be pitched down all that steeply to drop quickly and if you're falling at a steady rate, everything will be at 1G.

When you stand next to a fast road and a car goes past at 70mph it feels really fast. I guess that I never really "get" just how high planes are at cruising altitude

I guess it's only 3.5 miles or 20,000 feet in 7 minutes, and planes are a long way up.

It's pretty fast, but pretty well required because once you lose pressurization you want to get down to a normal pressure level quickly as staying at a high altitude is not particularly comfortable and everyone in the aircraft needs to have supplemental oxygen until you get under 10,000 feet. If anyone doesn't get a mask on they could be in trouble due to lack of oxygen.

This is a pretty non-event. There are a number of system failures that can cause loss of pressurization.

I've listened to a lot of ATC recordings and there are many engine fires, bird strikes, and general malfunctions that never get reported on.

Here's a video of two pilots demonstrating an emergency descent in a simulator: https://www.youtube.com/watch?v=yHawjB2PzK0

I was a passenger on AC549 several years ago when this happened. In our case, the oxygen masks were deployed because the cabin had actually lost pressure. From the standpoint of mechanical safety, I have no doubt that these planes are built to safely withstand these rates of descent, and likely a multiple of them. Having said that, I can also say that when the plane began its (sudden and unannounced, understandably) descent, you could feel your guts up in your throat much like when the roller coaster drops over the top of the first hill. From there on down it was calmer, but nonetheless harrowing. It's hard to describe how exactly, but it sure felt like we were coming down in a hell of a hurry.

Only once we had levelled off were the flight crew able to inform us what had happened. Those intervening 6 or 8 minutes, however, were decidedly Type 3 fun.

https://globalnews.ca/news/2487158/breaking-vancouver-bound-...

TIL: types of "fun"

https://www.rei.com/blog/climb/fun-scale

For me, an airplane depresurization event without structural damage would probably be a 1.5. Worrying/Stressing in the passenger seat isn't going to affect the outcome, so might as well sit back and enjoy.

Avherald: https://avherald.com/h?article=50e68713&opt=0

I believe something similar to this happened to me on a ~20 minute puddle jumper flight. Late at night, nothing unusual then out of nowhere we descend to what felt like just above the tree-line, as in low enough to make out the edges of license plates on cars below, even in the glow of the street lights. Very distressing, especially because the pilots didn't mention anything over the intercom before during or after. We stayed that low long enough for me to quickly call my close family members.

There wasn't any feeling of falling at all. Also absent of any feeling of revitalization after a near-death experience, if that is in fact what is was or could have been.

Then back up just under the cloud layer, landed fine and then that was that.

An emergency descent for pressurization stops at 9,000~10,000 feet or minimum safe altitude. You shouldn't be low enough to see license plates.

Sounds like a standard, run-of-the mill "dive and drive" style non-precision instrument approach. With the advent of commonplace GPS based approaches, they have become less common than they used to be, but they still exist and are safe to use.

Non-Precision means that you are using a simple ground-based navigation aid that does not provide vertical guidance, (a glide path indication) for example an NDB (Non-directional beacon) or VOR (VHF Omnidirectional Range) So, there are intermediate "step-down" fixes, that require you to maintain a certain minimum altitude until you reach a certain distance from the navaid. The route and altitude requirements (obstacle and terrain clearance) are selected by a well documented process called TERPS / FAA Order 8260.3F. (Edit, oops up to F now.)

There's always been debate among pilots about whether these approaches should be flow using a "Continuous Descent" method, where you select a descent rate to try to arrive at the altitude crossing restriction exactly before the next one comes into effect, or if you should descend rapidly to the altitude restriction, then level off until the next altitude restriction arrives, AKA "Dive and Drive."

There are pros and cons to each method and you can find well respected flight instructors that advocate for each. But a "Dive and Drive" approach can put you visually close to the ground when you are still pretty far from he airport. Really, modern GPS based RNAV approaches make the whole debate close to obsolete. Any podunk airport can get a satellite based precision approach with no significant investment in equipment, just a survey and some lighting improvements.

Title: United flight descends 28,000 feet...

General question: why is it that for such events, it is usually the airline name, or sometimes the flight number, that is mentioned first, and not the plane model? That would seem more relevant to me (not least considering Boeing's safety history).

Because most of the time the reason for an issue like this is maintenance, or pilot and/or crew error. Both of which are mostly guided by airline policy and training.

My calculations put that at a glide slope of around 3.6 degrees down.

For reference, the standard ILS glideslope angle for the final approach segment of every normal commercial airport is 3.0 degrees. But steeper ones are not uncommon, London City is an extreme at 5.5 degrees, and requires special aicraft/aircrew certification.

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