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Old 16th July 2016, 12:23   #286
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Default Re: Boeing 777 - Pilot's Review

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Originally Posted by im_srini View Post
Surprising thing is that it appears that only the bypass airflow is "reversed", at least from what's shown in the video. So, the airflow through the core ( gas-producer section ? ) would still generate thrust in the forward direction isn't it ?

When the switch is made from reverse-idle to full-reverse, the thrust from the core & the thrust from the bypass airflow oppose each other ( ? ).

Given the level of deceleration usually seen when the reversers come on, the bypass airflow must be contributing a significant percentage of the total thrust, isn't it ? It's surprising these fans generate so much thrust, given that there's just the one fan. What RPMs do these fans rotate at ? The internet says 2,500 to 3,000 RPM, is this correct ?
.
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Originally Posted by honeybee View Post
That seems to be correct, and a bit surprising - considering even a car's engine does about 6k rpms without sweat. I was under the impression these turbine engines could and would do about 30k rpms. But the blades are massive, so I guess that low RPM figure is still good enough to power the plane.
For the GE-90 115B, the bypass ratio is 9. In english, this means that 9 kg of air passes around the core for every 1 kg of air passing through the core.

N1, i.e the first stage fan turns at a maximum of 2,602 rpm. For a turbofan with a 128 inch blade, it's really high. The larger the diameter of the fan, the slower its maximum RPM. If you are watching an airliner taking off and you are to the front of the airplane, you will hear a sound similar to a large rotary blade saw cutting wood. That sound is the noise caused by the blade tip shock waves as they just exceed Mach 1. It is a bad thing to have supersonic fan blade tips. But in turbofans it is a price worth paying, because the faster tip velocity means higher dynamic pressure, and the pressure difference between both sides of the fan blade grows with the square of their velocity. This makes the high thrust levels of modern turbofans possible

60% thrust produced by the bypass air, whose airflow is reversed using a cascade vane. The core still produces thrust, but in the opposite direction. As Jeroes said, reverse thrusters aid in braking the aircraft. They are not the primary means of braking. That is provided by the carbon fiber brakes on the 777.

The following figure shows the schematic operation of the thrust reversers.
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Here are the maximum engine RPM's for the GE90-115B.
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Quote:
Originally Posted by Jeroen View Post
The actual bypass ratio varies a lot too, but again, rule of thumb for the typical wide bodies, somewhere in the 1:8 - 1:10 ratio

Note that in most landings it is the brakes that ensure deceleration! The thrust reversers are there for the brakes to do less.

Most commercial jet liners and certainly all wide bodies have a so called auto-brake system. It does so in a very clever fashion so it brakes at a constant rate of deceleration.

The auto-brake system is also used (armed) during take off. Simply put, if the pilots close down the throttles (move to idle) during the take-off, it is seen as a RTO (Rejected Take Off) and the auto-brake applies maximum braking power immediately.
To add to what you said, the RTO mode can be selected only on the ground. The RTO autobrake provides maximum braking pressure only if:

the airplane is on the ground
groundspeed is above 85 knots, and
both thrust levers are retarded to idle.

If an RTO is initiated below 85 knots, the RTO autobrake function does not operate.

Autobrake application occurs slightly after main gear touchdown. If MAX AUTO is selected, braking is limited to the AUTOBRAKE 4 level until pitch angle is less than one degree, then increased to the MAX AUTO level. On dry runways, the maximum braking in the autobrake mode is less than that produced by full pedal braking. To maintain the selected braking rate, autobrake pressure is reduced as other controls, such as thrust reversers and spoilers, contribute to total deceleration.

EDIT:

Today is the 16th of July, 2016. Boeing, turns 100 years old today. In the century since William E. Boeing took the fortune he made in timber and turned it to the emerging, thrilling field of aviation, his company has changed the way we look at the world. Here is a photo as a tribute to the same.

Boeing 777 - Pilot's Review-cncnsl_umaabkqz-1.jpg

Last edited by searchingheaven : 16th July 2016 at 12:31.
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Old 17th July 2016, 02:02   #287
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Quote:
Originally Posted by Jeroen View Post
Yes, the air through the core does still generate thrust.
However, the vast majority of the thrust is generated by the bypass airflow and it is only that part that gets reversed.
Thanks, looks like these engines operate more like ducted fans.
Reminds me of the ultra-high-bypass engines that were experimented on in the eighties.

Quote:
Originally Posted by searchingheaven View Post
For the GE-90 115B, the bypass ratio is 9, this means that 9 kg of air passes around the core for every 1 kg of air passing through the core.
N1, i.e the first stage fan turns at a maximum of 2,602 rpm.
For a turbofan with a 128 inch blade, it's really high.
Thanks for all the cool information, I was previously under the impression that bypass-ratio referred to volume, not weight.
It's surprising the fan blades exceed Mach 1 - at 2,602 RPM, the tip of a 128" fan is at 1.3 Mach !?!
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Originally Posted by searchingheaven View Post
60% thrust produced by the bypass air, whose airflow is reversed using a cascade vane.
The core still produces thrust, but in the opposite direction.
The fan generates 60% of the total thrust, but the direction in which the thrust reversers deflect the bypass airflow is only slightly forward.
One would think that the increased thrust through the core (40%) would be greater than whatever reverse thrust is generated by "reversing" the bypass airflow.
Quote:
Originally Posted by honeybee View Post
That seems to be correct, and a bit surprising - considering even a car's engine does about 6k rpms without sweat.
I was under the impression these turbine engines could and would do about 30k rpms.
It's only the fan at the front that's at a relatively low RPM.
Actually, to be specific, since the fan, the IP compressor (LP ?) & the LP turbine all share the same shaft, they would all have the same RPM.
The engine architecture comprises of co-axial shafts (with a compressor section & turbine section at either end) that are independent & hence can have different RPMs.
The GE90 appears to have a twin-shaft design, the HP compressor & turbine sections would have a much higher RPM (N2).
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Old 17th July 2016, 22:45   #288
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Originally Posted by im_srini View Post
Thanks, looks like these engines operate more like ducted fans.

).

They are ducted fans. Turbo props have a much higher ratio, 60 sometimes higher. Although it must be said that there is always endless debate on how to calculate a bypass ratio on a non ducted engine.

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Old 18th July 2016, 08:49   #289
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Watch the 2nd plane, an Antonov variant, with the Judgement day sticker under the cockpit. It is so massive, its wings have a supporting wheel under each of them, and two sets of nose wheels, but behind the nose.

If you observe how it taxis to the end, turns around and then takes off, the wing on the viewer's side has dipped and its wheel is on the ground, whereas the wing on the other aide is far up in the air and its wheel well clear of the ground. This reminds me of a car bonnet held up with the supporting rod at one end, which causes the bonnet to droop down at the opposite end.

Does something similar happen to planes?

Also which is the ugliest plane today? I watched the HUGE and HUGELY UGLY Airbus Beluga and that alien-head-on-human-body contraption should easily take the honours.
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Old 18th July 2016, 09:14   #290
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Quote:
Originally Posted by honeybee View Post
Watch the 2nd plane, an Antonov variant, with the Judgement day sticker under the cockpit.
The 2nd plane is a B-52 - specifically a Boeing B-52H from the 917th Air Force Reserve Wing based out of Barksdale AFB.
Quote:
Originally Posted by honeybee View Post
It is so massive, its wings have a supporting wheel under each of them, and two sets of nose wheels, but behind the nose.
Planes with long, thin, wings, which can be filled with fuel have such outriggers to prevent the wing tips from touching the ground.
Another famous plane that had these was the U-2 spy plane.

P.S. - Is there such a thing as an "ugly" airplane ?

Last edited by im_srini : 18th July 2016 at 09:16.
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Old 18th July 2016, 10:39   #291
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Yes, after watching the Beluga, I am convinced there's such a thing as ugly planes.
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Old 18th July 2016, 13:14   #292
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Originally Posted by honeybee View Post
Yes, after watching the Beluga, I am convinced there's such a thing as ugly planes.

The internet has a whole host of "most ugly" planes

https://www.quora.com/What-was-the-w...ft-of-all-time

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Old 18th July 2016, 17:04   #293
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Default Re: Boeing 777 - Pilot's Review

Just adding a bit more detail to the thrust reverse of, particular, by pass engines.

There is a misconception that the 'braking action' is due to the 'reverse air' being blown out forward. If you look closely to the design and some of the photograhps you will notice that on most by pass engine, the ''reversed air' gets blown out almost perpenduclar to the longitudital axle of the engine, or at best at a small angle forward.

A turbofan generates both a drag and a thrust. The drag comes from compressing and therefor actually slowing down air in the inlet. During normal operation the air being expelled at the rear delivers much more thrust then the drag, hence the plane moves forward.

During reverse operations the air is expelled almost sideways, so not much thrust forwards or reverse. But there is still a huge drag to content with. The drag comes down rapidly as the plane decellerates.

Reverse thruster typically gets stowed below 80-90 knots because they are not effective anymore and the likelyhood of it sucking up some debris (foreign objects) increases too.

So the braking comes from the drag on the engine, without creating forward thrust. Bit counterintuitive and for pilots as such, not relevant, as long they get the braking force, more a bit of anorak information.

Not sure if you can reverse a 777 on reverse thrust? On 747-400 it tends to end in a compressor stall, well at least on the simulator. Not sure on the real thing though.



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Old 3rd August 2016, 15:49   #294
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Default Re: Boeing 777 - Pilot's Review

Some news item today :: Crash landing of a Boeing 777

http://timesofindia.indiatimes.com/w...w/53522543.cms

More technical details are not in the media yet.

Regards
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Old 3rd August 2016, 16:00   #295
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Default Re: Boeing 777 - Pilot's Review

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Originally Posted by Dieseltuned View Post
Some news item today :: Crash landing of a Boeing 777

http://timesofindia.indiatimes.com/w...w/53522543.cms

More technical details are not in the media yet.

Regards
Dieseltuned
Oh well, that looks like the first hull loss for emirates? Or is it not? Quite shocking considering they had a good safety record all the while. Some more info here:
http://avherald.com/h?article=49c12302&opt=0

Quote:
An Emirates Airlines Boeing 777-300, performing flight EK-521 from Thiruvananthapuram to Dubai with 275 people on board, was on final approach to Dubai's runway 12L at 12:41L but attempted to go around from low height. The aircraft however did not climb, but after retracting the gear touched down on the runway and burst into flames. Passengers are being reported evacuated and safe. The aircraft burned down completely.

The airline reported: "Emirates can confirm that an incident happened at Dubai International Airport on 3rd August 2016 at about 12.45pm local time."

.
Quite a lucky escape there for the passengers. Touching down without gear is something totally amiss here. Even if they were going around, shouldnt they have retracted it later?

Last edited by audioholic : 3rd August 2016 at 16:04.
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Old 3rd August 2016, 21:04   #296
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Quite a lucky escape there for the passengers. Touching down without gear is something totally amiss here. Even if they were going around, shouldnt they have retracted it later?
Some thing does not add up, let's wait for an official report before speculating.

The standard procedure is that as soon as a "Go around" call is given by pilot flying, he gives TOGA (take off- go around thrust), pilot non flying says "positive climb" after confirming that a/c is gaining altitude, pilot flying double checks to confirm it indeed is a positive climb and then only calls "gear up", the aircraft gains a lot of altitude during this period unless the go around was being done at a ridiculously low altitude.

TOGA thrust has immense power but these huge turbo fans do need a few seconds to fully spool up from near idle thrust during landing, it is impossible that the experienced crew put the gear up without even confirming a positive climb rate unless some weather phenomenon like wind shear brought that aircraft down.
Yet to see what the weather was like and finer details on what happened, am glad that all passengers and crew escaped unhurt.
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Old 3rd August 2016, 22:58   #297
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I believe wind shear was prevailing at the time of crash. OMDB metar 030900z 11021kt 3000 BLDU NSC 49/07 QNH 0993 WS ALL RWY TEMPO 35015 kt1500 DU. That is weather Dubai winds are from 110 degrees direction at 21 knots with visibility of 3 km blowing dust. No significant cloud was reported and temperatures of 49 degrees.
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Old 4th August 2016, 14:21   #298
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In most websites, people are speculating about the cause of the crash. I am glad that T-bhp is more considerate towards the aviation community. Believe me when I say this, even as a pilot, I would still hesitate to judge the quality of a colleague's landing even when I am sitting in his plane. I could well say whether his landing was smooth or hard, but I would not dare to say it was good or bad, unless I am inside the cockpit and I am familiar with the plane and the respective procedures.

Also keep in mind that the conditions in the Gulf in summer are extreme, as you see. Last second go around in Dubai with WS is a very high risk scenario. With a prevailing temperature of 49 degrees Celsius and barometric pressure of 0993, both pressure and density altitude are high considering the actual field elevation of 62 ft. Coupled with Wind Shear Alert, conditions were quite adverse. However, please remember that I say this on the basis of the METAR, and as every pilot knows, the METAR gives a hint of what MIGHT have happened, but a METAR can be quite a bit different to the actual conditions which vary within seconds.

So once again, patience is advised, let us wait for crew reports and data readouts. This is not an aircraft that has disappeared in the middle of the ocean. Its FDRs are being recovered as we speak and the reasons of this crash will be out in a few days.

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I believe wind shear was prevailing at the time of crash. OMDB metar 030900z 11021kt 3000 BLDU NSC 49/07 QNH 0993 WS ALL RWY TEMPO 35015 kt1500 DU. That is weather Dubai winds are from 110 degrees direction at 21 knots with visibility of 3 km blowing dust. No significant cloud was reported and temperatures of 49 degrees.
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Originally Posted by audioholic View Post
Oh well, that looks like the first hull loss for emirates? Or is it not? Quite shocking considering they had a good safety record all the while.

Quite a lucky escape there for the passengers. Touching down without gear is something totally amiss here. Even if they were going around, shouldn't they have retracted it later?
Quote:
Originally Posted by coolboy007 View Post
TOGA thrust has immense power but these huge turbo fans do need a few seconds to fully spool up from near idle thrust during landing, it is impossible that the experienced crew put the gear up without even confirming a positive climb rate unless some weather phenomenon like wind shear brought that aircraft down. Yet to see what the weather was like and finer details on what happened, am glad that all passengers and crew escaped unhurt.
Exactly. I will absolutely refuse to believe that the gear was retracted without confirming a positive ROC.

The only thing I am not happy about is the attitude of the passengers after the crash and the subsequent evacuation. People were panicking, trying to take their hand-baggage and luggage etc.
  • Most people who encounter an incident like this for the first time are disoriented. They go into shock. They cannot think properly anymore. They know they need to get off the aircraft so they do what they normally do, grab their bag and head for the exit. You wouldn't believe how the human mind can freeze up. The seatbelt demonstration that everyone laughs at? There have been a number of incidents where people survived the impacts but died in the fire because they couldn't get their seatbelt off. Experts recommend buckling and unbuckling your seatbelt a couple times to build some muscle memory if you aren't familiar.

  • Most of the pax on board this aircraft was of an Indian nationality and it's safe to assume their English may not have been good. Also, cabin crew instructions were being shouted in English, most pax probably did not understand.

  • Another point to note, specifically in this scenario, is that most of the people in this flight would have been Indian nationals, whose passport or documents would mean more to them than their life. Here is a tip for everyone: always remember to keep your passport, visa and a govt. issued ID card on yourself at all times, even when going to the washroom. Not in your carry-on, not in your checked in baggage. On you, in the pocket or in a pouch or whatever. Always.
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Old 4th August 2016, 14:57   #299
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Default Re: Boeing 777 - Pilot's Review

And yet.despite all that, still they cleared the plane, the cabin of which was almost totally destroyed by the fire only a short time later, without fatalities. Miraculous! Or fantastic work by the crew.

I have to confess that I know that I would find it very hard not to pick up the small handbag containing the items you mention as well as one or two other valuables, even though I know I should not. But getting larger carry-on cases from lockers? I sympathise but Noooooo!

It may be insignificant, given that they have their lives, but these people will have suffered loss. In some cases, it might have been considerable. I don't suppose anything in the hold would have survived?

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Old 4th August 2016, 14:59   #300
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Default Re: Boeing 777 - Pilot's Review

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Originally Posted by searchingheaven View Post
In most websites, people are speculating about the cause of the crash. I am glad that T-bhp is more considerate towards the aviation community.

Here is a tip for everyone: always remember to keep your passport, visa and a govt. issued ID card on yourself at all times, even when going to the washroom. Not in your carry-on, not in your checked in baggage. On you, in the pocket or in a pouch or whatever. Always.
True, you only need to visit pprune.org and other sites to see the discussions. Anyway with Donald losing it its only a matter of time before he blames Hillary for this too

You are also spot on with your advice on carrying one's passport at ALL times. I practice it religiously even though it makes me the butt of jokes among my fellow travellers. It REALLY doesn't take much - just plan on wearing a comfort fit cargo pants/shorts with (ideally) zipped multiple pockets.

Pop in your passport, a portion of your TCs / forex notes, a mobile phone and any connecting/return flight tickets and you're done. At LEAST, these will allow you back into your home country without any fuss.

Last but not the least, for frequent travellers with multiple valid visas to any western country, just think of the time and money you will incur to replace the visas. Passport can be pretty easily done in 1 week once back in India. But the VISAS?!
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