I was there when the DC-8 went supersonic | Hacker News Reader

I was there when the DC-8 went supersonic

airspacemag.com

131 pointsareoform41 comments

A 727 once went into a dive due an autopilot malfunction. The separation of airflow over the stabilizer meant it was ineffective. The pilot recovered by lowering the landing gear. It tore the doors off, but slowed the jet down enough that he could regain control. I don't know if it went supersonic or not.

If you're not careful with a jetliner, it can go into "mach tuck" if you're in a shallow dive, where (again) the separation of flow over the stabilizer makes it ineffective. In one incident on a 707, the pilots weren't paying attention and it went into a shallow dive, and mach tuck. The elevator was ineffective, and the stab trim system wasn't functioning (I don't recall why, but likely it was overloaded). So the copilot got down on his needs and cranked the manual stab trim wheel, and got control back just in time.

(The 707 and 737's had a manual stab trim wheel, as does the 737MAX.)

Edit: Well, well, another poster linked to a wikipedia article about the 727 incident https://en.wikipedia.org/wiki/TWA_Flight_841_(1979) so it seems my memory was accurate.

727 has a high horizon stab which means that this can also happen in a stall - the stab is in the disrupted airflow from the wings

Autopilot malfunction is too charitable to the pilots. From wiki, "Investigators believed that 727 pilots (in general, and this flight specifically) were setting the trailing edge flaps to two degrees during high altitude cruise, while at the same time pulling the circuit breaker for the slats so that they would not activate."

The captain denied that, but who can believe him? He erased 21 minutes out of 30 minutes recorded from the cockpit voice recorder.

> He erased 21 minutes out of 30 minutes recorded from the cockpit voice recorder.

The investigation never concluded this.

In a deposition taken by the Safety Board, the captain stated that he usually activates the bulk-erase feature on the CVR at the conclusion of each flight to preclude inappropriate use of recorded conversations. However, in this instance, he could not recall having done so.

Are we talking about the same investigation?

The NTSB made the following statement in the accident report:

We believe the captain's erasure of the CVR is a factor we cannot ignore and cannot sanction.

Airplane accident causes are often complicated. I was told the story by my lead in helping me to understand separation. The reason for the dive was not germane, he just said autopilot malfunction.

Probably the funnest part about working in engineering at Boeing was hearing all the war stories from the previous generation.

Something similar happened on early Bell X-1 flights before they realized they had to modify the tail.

Supersonic airflow does weird things, and tends to separate from the horizontal stabilizer well ahead of the control surfaces on the trailing edge. Early transonic aircraft would go supersonic in a dive, wouldn't have the control authority to get out of it, and would careen into the ground.

They eventually figured out that the entire tail needs to be the control surface if they wanted it to remain effective beyond mach 1.

I thought this would have been well known by the time the DC-8 flew, but perhaps not, because if it had been, nobody in their right mind would have attempted a stunt like this.

Your comment matches common anecdotes, but I can provide more detail (I study aerodynamics.)

The background is:

- Britain sent data on the Miles M.52 supersonic jet to the US, so at least some Americans knew about the flying tail before the X-1 (first supersonic flight in 1948), and the Germans did some work on flying tails

https://en.wikipedia.org/wiki/Miles_M.52

- most of the pre X-1 planes that crashed didn't reach Mach 1, but they did lose control. Those are called loss of control due to compressibility, not supersonic flights.

- as the article says, the DC-8 flew ok up to Mach 0.97 in previous flights, so they thought things were fine. Note that in every Mach 1.0 flight that I've read about, the airspeed indicator jumps from around Mach 0.96 to 1.0, perhaps meaning the indicator is not accurate around Mach 1.0.

You have to correct for the shockwave that sits in front of the pitot static probe. Otherwise you get crazy results.

It's crazy how advanced some UK technology was back then. Designing afterburners in 1943. And then canceling just before completion of the aircraft.

The same happens in many cases. What's a successful breakthrough is usually not only technological. UK was extremely indebted after the second world war, mostly to USA. https://www.exchangerates.org.uk/articles/1325/the-200-year-...

Of course there was the wider sad story of management problems in UK industry and the long decline after the war.

I grudgingly give credit to the British for being very clever (magnetron, battle of the beams, etc.) during and after WW2.

Having said that, London had food rationing for 9 years after the war, and really didn't have the money or facilities that the Americans had. (A B-29 was used as the mothership for the X-1!)

The W article below mentions "After the rocket plane experienced compressibility problems during 1947, it was modified with a variable-incidence tailplane following technology transfer with the United Kingdom."

https://en.wikipedia.org/wiki/Bell_X-1

Additionally, around half the X-plane pilots died, so Britain avoided a lot of test pilot fatalities. The US poured unlimited funding and talent into the program, with everybody being stars/aces (Yeager, Hoover, etc.) Both were injured seriously and almost died in various X-plane tests.

One X-plane test pilot set a record, then just quit and started a gun range.

The UK is also the only nation to successfully produce an orbital-class spacecraft then cancel their space program.

Our inability to follow through in recent decades can be depressing.

Hopefully SABRE/Skylon doesn't turn out to just be vaporware.

The German Me-262 (jet fighter) had regular elevators, and if flying one you have to be very careful not to overspeed it because of separation.

Precisely. It also makes me wonder if they really reached Mach 1.01. I mean if you can recover with such an airliner, why did everybody before chuck Yeager die?

Jetliners differ from old prop planes in that they do have a completely moving stabilizer, like fighter jets. But it's used for trimming, not for fast maneuvers. It has the regular elevators for faster maneuvers too.

As an aside, most modern sailing boats tend to have rudders like this, called "skegless" or "spade" rudders. Though AFAIU this is to simplify construction, and rudders with a skeg are supposedly better.

Edit: after re-reading the passage, I think that what happened is that the elevator had stalled, not the wing? So it wasn't a "normal" stall. Original comment below.

> It stalled, because of the load. What he did, because he was smart, is something that no other pilot would do: He pushed over into the dive more,

This part made me chuckle. The fact that you need to go nose down to recover from a stall is one of the first things new pilots learn[1], it's not "something no other pilot would do", but I get it, they're trying to make the story interesting for the general public.

[1] https://www.faa.gov/regulations_policies/handbooks_manuals/a...

Also pushing the nose down is the first thing you're taught as a glider pilot for engery management: launch failures, stalls, spin recovery, etc.

Flight Chops did a video on this - it's drilled into commercial pilots (as well as glider pilots), but not GA nearly as much

Think it's this one: https://m.youtube.com/watch?reload=9&v=m_tKShlf_gU

Wouldn't take advice from that guy https://www.avweb.com/news/pilot-arrested-after-airport-inci...

> is the first thing you're taught as a glider pilot

I love gliders, I grew up near a glider school, near Rome, Italy. I live in Australia now and it seems that glider culture is not as strong as in Europe.

> it's drilled into commercial pilots

Probably even more after Air France flight 447

> but not GA nearly as much

Really? It was one of the first things I learned as I was studying towards the PPL (I haven't finished it yet because flying is expensive :-( but I'm done studying the theory).

I'm in Australia BTW, not in USA - maybe things are different

Well, you’re trained to push the nose down in a stall because stalls normally happen at a very high AoA, when there is not enough airflow over the wings, typically when you’re pointing up, not when you’re in a dive, and certainly not when you’re in a Mach 1.01 dive. And this was an elevator stall.

You’re not taught to pitch down when there is no elevator authority - in fact you wouldn’t be able to, by definition. But the problem here was too much airflow over the horizontal stabiliser - from what I can see, it hadn’t really stalled. Pitching down relieved the load and let them move the stabiliser. I suppose the combined deflection of the stabiliser and then the elevators was enough to get it under control again.

I for one am always pretty impressed with stories like these because they show just how well the pilots understand their aircraft.

> But the problem here was too much airflow over the horizontal stabiliser

Yes, after thinking about it, maybe they shouldn't have used the word "stall" because in this case it's a bit ambiguous

I would assume they were talking about the stabilizer trim motor not being able to run the stabilizer trim because of the loads.

> When he pulled back, the elevator was ineffective; it didn’t do anything, so he said, “Well, I’ll use the stabilizer,” and the stabilizer wouldn’t run. It stalled, because of the load.

Key in there: "Wouldn't run."

Almost all transport class aircraft have the same sort of trim setup on the tail - they have the movable elevator at the trailing edge, and then the entire horizontal stabilizer can tilt up/down for trim. This is a lot lower drag than having the elevator deflected, and gives you an extra control surface for emergency use as well - usually, the control authority of the entire moving stabilizer is more than the control authority of the elevator, so you can use that in an emergency for pitch.

That the elevator wouldn't work effectively in the transsonic region was known - that's why they planned to use the whole moving stabilizer to pull out of the dive. Just, the air loads on it were too high with what limited authority the elevator still had, so they had to unload it again to run the trim, at which point they were able to recover.

The term "stalled" is overloaded an awful lot in aviation, but I'm pretty sure in this case, it didn't refer to an aerodynamic stall of the tailplane. There's virtually no way it could be aerodynamically stalled at ~Mach 1 without ripping pieces off.

Yes you are right it was the stabilizer not the elevator. I knew that airliners have a stabilizer, but I wasn't familiar with how it functions since I've only ever studied in relation to small planes, thanks for adding some details.

And yes I think it was a "stall" in quotes, not a stall ;-)

> But the recovery was a little scary. When he pulled back, the elevator was ineffective; it didn’t do anything, so he said, “Well, I’ll use the stabilizer,” and the stabilizer wouldn’t run. It stalled, because of the load. What he did, because he was smart, is something that no other pilot would do: He pushed over into the dive more, which relieved the load on the stabilizer.

So. Nuts.

Note that they also took off with broken slots, which prevented them from extending the flaps, unknowing why these were broken. BUT STILL WENT.

For whatever reason I believe this wouldn't fly (ahah) today, too much risk.

Definitly would not fly today, there is a list called the MEL which specifies exactly when you can and cannot fly. Something like this where an engine failure would be unrecoverable would be an absolute "no fly" item.

The MEL (minimum equipment list) applies only to normal operations.

Test flight programs are generally done in the experimental category, where the MEL would not apply.

Side note: production airplanes are also regularly ferried in violation of the MEL or other airworthiness requirement under a ferry permit (Special Flight Permit is the official name) - https://www.faa.gov/about/office_org/field_offices/fsdo/phl/...

> something that no other pilot would do

I doubt that. A military fighter pilot would do that. Military fighter pilots are not afraid of diving.

My dad (KW fighter pilot) told me his favorite method of attacking ack-ack guns was to dive straight down on them. Anti-aircraft gunners do not want to fire straight up, for obvious reasons. He said he'd keep one eye on the altimeter, one eye on the airspeed, and his third eye (!) on the target. The trick was to keep the airspeed below the maximum and leave yourself altitude to recover. He also got yelled at once for flying a B-17 like a fighter.

The other two methods of attack are diving out of the sun (just put your airplane's shadow on the gun emplacement) so they can't see or hear you, and hedge-hopping off to the side. This makes it a very difficult deflection shot to hit you, and if you're close enough the gun cannot swivel fast enough to get a bead on you, then turn hard at the last second, line up, and blast it.

In fact, I think most pilots would have done that.

Pilots are trained to recover from stalls over and over, until it becomes second nature. And the way to do it is to follow into the dive until you regain control authority and then pull back slowly. This is not a regular stall but the idea is the same: pulling doesn't work? try pushing.

Of course I wasn't in the cockpit, and I would probably be panicking if I was, but from my armchair, it looks like the kind of reaction a properly trained pilot would naturally have.

With the arrival of jets, airliners required a movable stabiliser to accommodate CG changes as fuel was consumed from tanks in swept wings (later on fuel was put in the stabiliser in some models).

Of course the supersonic program led to the development of the movable stabiliser.

Typically an airliner stabiliser is moved with a screwjack. Air loads can overpower the screwjack;so, you have to relieve the air load enough to allow the screwjack to do its job.

Next to none of this applies to smaller aircraft which have a variety of trim systems, mostly single axis.

Quote: "And John Londelius, VP of Flight Test, gave us each a $1,000 bonus, so that was rather nice. That was back when a thousand dollars was worth a thousand dollars."

That got me curious and got that in today's dollars that's ~$8750. Indeed back then a thousand dollars was worth a thousand dollars :).

10% of the median price of a house then vs less than 2.5% today. Inflation isn't always a great measure for comparison, especially as over longer periods of time it starts to get decisively apples to oranges.

Hey we didn’t know we wanted oranges back then

I think it's better to look at certain items to understand the value. For instance, a brand new Ford Mustang back then was $2300 when it launched. So in that context, getting a $1000 bonus is very nice indeed.

The only time an airliner intentionally went supersonic besides the Concorde and Tu-144. Cool read.

I believe there are a few unintentional cases ... https://en.wikipedia.org/wiki/China_Airlines_Flight_006 and https://en.wikipedia.org/wiki/TWA_Flight_841_(1979)

Possibly the 737 MAX 8 aircraft that crashed as well, but I'm not sure it was confirmed.

Those weren't supersonic, but they were above the mach limit for the aircraft. Which is dangerous because it could cause severe flutter and break control surfaces or worse.

No many people realize that jets have been slowing down since the 707/DC-8. And this has been intentional as a trade against fuel efficiency. Both the 707 and DC-8 were nearly transonic by design.

> Boeing will never try it [with the 707] because they don’t want to be second

Yeah, but Tex Johnston had already rolled the 707 prototype back in 1955 as well. I think both companies had the good sense to not keep pushing the limits of showmanship with transport category aircraft.

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