snowfalcon2

I agree fully with Chris. This is based on a survey of about 50 photographs of landing B777s showing the wing undersides. I can find only one picture possibly suggesting wing surface frost in the tank area (ANA-Star Alliance plane).
I would imagine the airflow in most cases prevents frost (or ice) to form until the plane has landed and slows down. The wing underside is also a high pressure area, meaning that moisture is less likely to condensate on that part of the wing than on a low-pressure area.

That said, most of the pictures are taken in conditions of less humidity than the weather BA38 had.

But if it's frost that is visible on the BA38 picture, it might be a fairly unique occurrence?

Here are some pictures:
http://www.airliners.net/photo/Korea...-ER/1341078/L/
http://www.airliners.net/photo/Briti...-ER/1339905/L/
http://www.airliners.net/photo/Air-C...-LR/1342818/L/
http://www.airliners.net/photo/Conti...-ER/1339702/L/
http://www.airliners.net/photo/Jet-A...-ER/1338232/L/
http://www.airliners.net/photo/Star-...-ER/1336739/L/
http://www.airliners.net/photo/Briti...-ER/1333455/L/
http://www.airliners.net/photo/Briti...-ER/1326063/L/ G-YMMM
http://www.airliners.net/photo/Korea...-ER/1325100/L/
http://www.airliners.net/photo/Korea...-ER/1321846/L/

EDIT: Based on some more searching, frost on the underside is not totally uncommon but still fairly rare. A search turned up a total of 3 pictures with visible frost under the tanks.

http://www.airliners.net/photo/Finna...-11/0991454/L/
http://www.airliners.net/photo/Virgi...443/0764483/L/
http://www.airliners.net/photo/Gulf-...101/0701662/L/

Green-dot

To Chris Scott and snowfalcon2:

I've checked A.net pictures as well today and came across almost the same pictures snowfalcon2 posted. I agree with you that actual fuel quantity is probably somewhat more than the frosted area for the reasons you explained. Making it only more likely there was sufficient fuel in the tanks before the moment of impact.

For it being a rare condition to observe frost on the lower wing surface, that may be when searching photographs for examples. But since I've performed countless post flight inspections and also participate in de-icing activities as a supervisor in winter seasons I come across frosted lower wing surfaces quite regularly if the weather conditions are right for it. Indeed the fuel level in the tanks usually exceeds the frosted area somewhat which can be observed by a wet thawed area bordering the frost at + deg. C conditions. This condition cannot be determined from the G-YMMM subject photograph but was probably present.

Therefore, I do believe the fuel quantity in G-YMMM's case was slightly more than the frosted area in the picture and will take it into account when making any calculations. However, I'm still convinced that the center wing tank was empty when observing the clear outline of frost starting at the rib, separating wing tank(s) from center wing tank. Also because of the absence of frost near the wing root.

Green-dot

DozyWannabe

Yes, more powerful hardware is available now - but do you know *why* the decision was made to use older hardware, and indeed why previous generation hardware continues to be used in safety-critical situations?

In the home and business sectors of computing, the onus is on bang for buck - no-one particularly cares if a fast processor crashes once in a while if the worst that can happen is losing a report that's due in tomorrow. In safety-critical systems the focus is on predictability - knowing that barring a catastrophic hardware failure a certain input will *always* produce a given output.

In the case of the Airbus system, the 80186 was a 1982 update of technology that was released in 1977, and had been developed for several years prior to that. Which means that it was in effect 6 years old by the time the A320 was delivered (26 years old now), based on technology that was 5 years older than that (31 years old now). Then as now, 6 years is a long time in microprocessor development.

Processors of that vintage were very simple technology by today's standards - but that's what makes them useful in real-time, safety-critical systems. As a Software Engineering graduate you'll remember your first Hardware Architecture lesson and the diagram you were shown, illustrating the program counter, registers and arithmetic/logic unit. '70s and early '80s era processors were slightly more complex but still hewed to that basic design. Today's modern processors are nothing like that - they are essentially a CISC translation unit which takes the x86 instructions and translates them into a format that a highly-complex RISC back-end then uses to do the grunt work. Such a design is wonderful for today's desktops, workstations and servers but their complexity makes it a challenge to answer the safety-critical question of predictability with any conviction.

Hence the A330/340 was introduced 4 years after the A320, but still used the 80186 in the SEC. The Space Shuttle continues to use heat and radiation-hardened variants of the Motorola 68000, a design of similar vintage to the original 8086 (but logically structured more in line with the PDP-11 computers used in development in the 1970s). The latest Honeywell FMS uses an AMD design that's 20 years old now.

Anyways, that's how I understand it - I'm sure the Prof will correct me if I've made any glaring mistakes, seeing as my knowledge was only current when I graduated Uni 7 years ago.

Apologies to the mods if this post is too far off-topic.

PAXboy

Dozy, not off thread, wonderfully ON thread to straighten some of the wonky lines that people have been shooting in here.

I am sorry that PBL cannot continue (for whatever reason) and, whilst remaining fascinated by this mystery, I think that I shall wait the results that RTFM has kindly calculate at 25.6 months. My thanks to RTFM for that, it confirms what I have said so often.

Many can now draw a sigh of relief as I leave this thread to it's natural end ...

LurkerBelow

Dozy: slight correction - Shuttle flight computers are actually made by IBM and the architecture is an update of the S/360 developed in 1960s (and is still in use today albeit much enhanced on the IBM mainframes).
The actual processors had a well tried pedigree as an avionics processor in such aircraft as the B-52 (obviously the later variants).

CONF iture

I believe the main idea is:
there was actually cold stuff (hopefully fuel) in these main tanks, and obviously enough for another 2 miles.

But still, I’ll be interested if Green-dot is able to produce an estimate.

I would not bet on any figure, but to me, the area looks pretty tiny, at least more tiny that the one from ANA when blue sky (and Metar confirms temp / dew point) was not optimal for that frost formation. To have access to that ANA remaining fuel could provide some direction of thinking …




Regarding BA038, humidity was effectively more important with that broken at 1000 ft confirmed by a 2 deg dew point spread on surface.
I do agree that TAT has an influence, but not that much in this case when you consider this remark:
“clear outline of frost starting at the rib, separating wing tank(s) from center wing tank”
Nevertheless I do consider the limited cold fuel influence on the outboard as volume is lesser, but I still find the outline quite marked.

Just one more thought:
Considering the huge size of the CWT close to 100000 liters (and also the architecture, as I understand it the tank is not in direct contact with the fairing skin ?) I’m not sure even 10000 liters would be made visible by any frost on the left or right side of that tank, as that volume would be exclusively in the lower part, just in the center of that center tank … ?

Swedish Steve

CONFiture

The wing fuel tank on the B777 goes a long way inboard of the strut.

I can't comment on how often frost is seem while aircraft are landing, but I do meet B777 every day on the ramp at ARN. On arrival on the ramp there is nearly always frost under the wings like on the picture above. The area under the wings inboard of the engine normally shows a clear line where the wing tank ends. Frost under the wings on long haul aircraft is there on arrival at the ramp about 90pc of the year. The air must be very dry for it not to form.

DozyWannabe

LurkerBelow:
Aye, you're probably right. I think the 68k was used in the engine control system rather than avionics.

kick the tires

A-320 is a totally different kettle of fish but, if I DONT see under surface wing frost on a turnround, I think something is wrong!!

Its a pure case of cold metal descending into warm air and picking up the moisture, its not an issue at all.

Chris Scott

Just how closely the under-wing frost defines the ‘wet’ part of a wing tank is a matter for some conjecture. I don’t disagree with anything that’s been said; nor amend my suggestions above.

Swedish Steve is our most experienced B777 witness on the ramp, and has stated that the inboard edge of the tank is clearly discernable. This is, of course, where the fuel is deepest; and the massive rib that forms the end of the tank has itself been refrigerated by the fuel for many hours. At the opposite end of the tank (the end we are considering), the shallow fuel is lapping the shore, so to speak; and not contained by a rib in the presumed fuel state.

CONF iture has reminded us of the centre-section (belly) fairing, which on most types is deepest at the forward end. My dial-up connection discourages me from downloading snowfalcon2’s excellent photos of the B777. But other types have enough depth to accommodate the air-con packs at the front. Further aft, the forward base of the centre tank is also well within the fairing. For example, tank-pump canister-plates are only accessible via an access panel on the fairing; likewise the magnetic fuel-level-indicator sticks for the centre tank.

snowfalcon2, could you tell us the extent of the B777’s belly fairing?

Subject to the above, I don’t think absence of frost on the belly fairing would tell us anything about centre-tank contents. As for the wing tank, see my post above. To paraphrase what I said: presence of frost indicates presence of fuel; conversely, absence of frost does not necessarily define precisely where the fuel is absent.

Swedish Steve

Took this this afternoon. A B777-200ER of Malaysia airlines just parked on the ramp at ARN after an 11 hour flight. This frost cover on the lower wing is typical. Total fuel on board 9700kgs.

Oh dear picture did't work. Will try again later.
Thats better.

snowfalcon2

I'm not sure if I understand the question 100%. I believe there are several posters here with a much better knowledge of the B777 tank construction than myself.

What I can add is the regulatory requirement. FAR 25.963 "Fuel tanks: General" states that:
To me, this means that the centre tank is inside the structure and thus frost formation on the outside of the belly due to cold fuel is highly unlikely. Maybe our Swedish contributor can confirm that.


Agree, see above. However, the centre tank does reach into the wings, as this incident report shows, so in theory frost could form on the wing underside portion of the centre tank if it contains fuel.

It is interesting that while underwing frost is commonplace on parked airplanes, judging from the photos it's quite rare to form while the plane is airborne, as it did on BA38. How rare, is an interesting question.

CONF iture

Very good Steve !
As frost shape is quite similar to what we saw under MMM, I think it does validate the "indicated" 10500 kg from AAIB report.
Thanks

GemDeveloper

A bit off thread, but...

... jolly nice to see an MH arrival with a bit of spare fuel on board...

Swedish Steve

If you look at my picture, the centre tank starts where the frost ends, and goes right across the aircraft. The wing/body fairing covers the centre part of it completely.

Machaca

Green-dot

Quoting CONF iture:

And Swedish Steve:

Very similar to the frost area under MMM, see picture and drawings below:










With special thanks to a fellow PPRuNe-er for helping me get wiser on how to post images here.

Checked another B777-200ER here at AMS during a post flight insp. which had approx. the same area of frost under the wing.

Regards,
Green-dot

DozyWannabe

So in layman's terms we can put the "poor fuel management" theory to bed with the EMI nonsense - is that the case?

Oh, and another thing I forgot from my earlier post - older processor designs didn't use as much power and didn't require large fans to keep the temperature under control - in engineering terms fewer parts = fewer things to go wrong.

autoflight

I am very interested to know all the differences between advancing the B777 thrust levers vs advancing them to maximum.

cats_five

I suspect we could ever since one of the two AAIB reports commented that indicated fuel was more than adequate.