Pilots of STW - hel...
 

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[Closed] Pilots of STW - help please

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I need your help.  I’m struggling again, already really stressed about flying to the US - Baltimore - on Saturday yet my anxiety levels have just soared as I listened to the news re Storm Ciara and super viscous Jetstreams.  I’m taking off at 3:30pm Heathrow and it’s supposed to hit 6:30pm onward.   The West/Northerly track will take me right through/past Ciara.

I need to know in all honesty if Ciara or the Jestream issue means I’m in for a rough ride, as I really don’t want  this flight to ruin the family hols later in the year by causing a refusal.

id appreciate your opinion of the issue, I’m stressing big time.


 
Posted : 05/02/2020 6:59 pm
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Take my kids with you.
Last time we had some serious turbulence, they shouted “again again” when things calmed down 😂


 
Posted : 05/02/2020 7:04 pm
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Unfortunately this isn't a joke for me, I'm barely functioning right now.


 
Posted : 05/02/2020 7:08 pm
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Before I flew to the US in 2018 I watched this video. I'm not convinced she's as "cured" as she claims but some of the stuff does help. I think she likes Amigo more than here partner...


 
Posted : 05/02/2020 7:15 pm
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US bound I think the flights go a bit further North to avoid the drag caused by the jet streams.


 
Posted : 05/02/2020 7:32 pm
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Is anxiety something that has come up in your therepy? Seems like you are always worried about something. Last week your trigger was worries about Boeing aircraft safety.

If you want to be calmed down, look at how many flights have successfully navigated this weather front since you posted.


 
Posted : 05/02/2020 7:44 pm
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Airlines are massively risk averse. If there was even a small risk from the storm, the airline would re-route the flight or even cancel it.

Easier said than done I know but try to relax and let them do the worrying on your behalf.


 
Posted : 05/02/2020 8:02 pm
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Your problem will be wind near the ground..if it is too strong you won't be aloud to take off.
Once you are airborne and clear of the first few hundred feet no worries.

I would not get stressed.


 
Posted : 05/02/2020 8:11 pm
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I think much bad weather is radar-visible, and they'll head for the smooth stuff as much as they can.


 
Posted : 05/02/2020 8:28 pm
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Absolutely no reason to be any more worried regarding the Ciara storm. Weather looks fine on Saturday afternoon when you're due to leave and then once you're airborne you'll probably miss most if not all of the jet stream. The only reason it's going to cause an issue is because it's moved south, but there is always a jet stream of some description over the Atlantic.

And to be honest, because it will just cause a long flight time because of the head wind, then I imagine most west-bound flights will be planned fairly far north at the weekend in an attempt to remain on schedule.


 
Posted : 05/02/2020 8:32 pm
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I used to hate flying, but don’t mind now - in fact I generally enjoy it. I found visualisation to be a big help ie visualising the aircraft successfully completing its journey without drama. Also the realisation that actually modern aircraft are much more robust than most people realise!


 
Posted : 05/02/2020 8:34 pm
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When I first read this thread and the title I assumed that you were flying yourself and your kids to the states in some kind of small aircraft. I thought "that's cool" and then "don't trust this guy if he's asking for advice on this forum about such a big thing." Then I realised you are just scared of flying.


 
Posted : 05/02/2020 8:51 pm
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The routes over the ocean change daily based on wind/weather. They try to plan to maximise tail winds / minimise headwinds and avoid weather. You’ll be fine.

Very few planes fall out of the sky due weather, and most of those are when they’re near the ground.

Enjoy your holiday 👍🙂


 
Posted : 05/02/2020 8:51 pm
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Hi. As previously stated the route used by aircraft to the US heads up to near Greenland and down over northern Canada to Baltimore (I was there in October). You shouldn’t have an turbulence issues and even then the weather radar on the planes mostly avoids bad weather. I’ve flow to the US twice last month (sorry Greta) both times as storms were coming in here and had nice smooth flights.

If you get a chance to wander round Baltimore - go have a look at the Liberty bell - it’s iconic but it’s cracked because it was a crap casting!


 
Posted : 05/02/2020 9:05 pm
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Turbulence affecting aircraft is typically a result of differing temperature air masses, not wind speed. The jet stream forecasts suggest it will be straight across the Atlantic, but planes fly on the great circle route which goes North of the straight line, so will miss it. I wouldn't expect your flight to be any different to usual.


 
Posted : 05/02/2020 9:19 pm
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If you get a chance to wander round Baltimore – go have a look at the Liberty bell – it’s iconic but it’s cracked because it was a crap casting!

That's in Philadelphia, innit?


 
Posted : 05/02/2020 9:38 pm
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If you get a chance to wander round Baltimore, don’t.


 
Posted : 05/02/2020 9:40 pm
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If you get a chance to wander round Baltimore then you landed safely. Result!


 
Posted : 05/02/2020 9:50 pm
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When was the last time that a weather storm took down an aircraft. Seriously, it just does not happen.

I hate flying, refuse to do it for work, and need pills before I go on a plane. But weather wouldn't bother me at all.

2 things to remember..the pilot doesn't want to die either.

And..you are infinitely more likely to die in your next xc mtb race..statistically..


 
Posted : 05/02/2020 9:53 pm
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Ha - sorry mis-read. Baltimore! Stick to the harbour area if you are downtown and don't wander. Detroit-on-sea (and I've spent a lot of time in Detroit) was my opinion of Baltimore last time I was there!


 
Posted : 05/02/2020 9:55 pm
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you are infinitely more likely to die in your next xc mtb race..statistically..

Now I want to know how many deaths there are in XC racing compared to flying....


 
Posted : 05/02/2020 10:05 pm
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Hi hate flying too but do it so often i just find it uncomfortable. But the odd thing is the flight tends to be smoother in wind storms. I took off in a hurricane once from Philadelphia and felt very little. Tend to get more turbulence in clear days and hot weather.
As others have said they’ll be hundreds of other planes doing the same route and they’ll land with no issues.


 
Posted : 05/02/2020 10:18 pm
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FWIW, one of the main reasons why aviation is statistically so safe is the fact that any avoidable human error is because of the litigious nature of the industry. That's a good thing, it ensures that air crew understand things like checklists and to be risk averse.

An irrational fear of flying is perfectly okay - there's some excellent tips from other posters. I agree with the visualization thing, maybe give it a go?


 
Posted : 05/02/2020 10:28 pm
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My Brother flies for a major airline, I've sat in the jump seat as we crossed the Atlantic to the US. The mitigation and effort that goes into ensuring a smooth flight is quite something. Before the flight takes off they plan the route carefully and have regular met updates en route (from a tiny fax machine!). The weather radar on board can detect any poor weather and the other aircraft report turbulence and what altitude to climb or descend to. As my brother says, turbulence in an aircraft is just like a pothole in a car. Please be assured (I appreciate this is major for you and don't want to dismiss your fears out of hand) your flight will go smoothly and you'll wonder why it was built up so much before hand.


 
Posted : 05/02/2020 10:29 pm
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https://i.pinimg.com/originals/ac/2c/34/ac2c348ad9a1552a6034d754c52ca57e.jpg


 
Posted : 05/02/2020 10:40 pm
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You'll be alreet.

I've flown to BWI a few times last year and you'll route north of the jet stream toward Greenland.

The company will aim to avoid significant turbulence too when they plan the route, however sometimes you'll hit the odd patch of rough air once in a while. The jets are tough and are designed to take a bit of a beating.

If we hit turbulence, we'll slow down to smooth things out a bit and potentially change level to either get out of the rough air, or give us more lattitude with our critical speeds at lower levels.

Weather radar will pick up moisture which indicates likely turbulence that we can avoid, but can't see clear air turbulence. The US pilots are OBSESSED with "ride reports" to the point of being annoying, but if it's lumpy, you'll get plenty (too much) feedback on the north atlantic common frequency 😣 so we're well prepared!

Coming back, you'll route further south to take advantage of the jet stream. I had a 170kt tailwind the other day heading to the middle east. Smooth as a smooth thing.

Crossing jetstreams sometimes is necessary and that can get a bit lumpy as you enter and exit the band of fast moving air; normally fine within.

Enjoy!


 
Posted : 05/02/2020 10:43 pm
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I took off in a hurricane once from Philadelphia

Cool.

Shame it has been so overlooked in favour of the more photogenic Spitfire.


 
Posted : 05/02/2020 11:40 pm
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Seriously Krypton, you’ll be fine. Flying in the jet stream is commonplace. The turbulence comes from passing through one body of air moving in a direction and at a speed, into another body of air moving in a different direction and a different speed. The air at the interface is stirred up and turbulent. Once the aircraft has passed through this thin layer and into a uniform layer of air all is smooth again. Passengers don’t like turbulence so aircraft will avoid it.


 
Posted : 06/02/2020 12:02 am
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The most dangerous aspect of your journey is Baltimore, as said above stick to downtown. Don’t wander about, don’t go West.


 
Posted : 06/02/2020 7:24 am
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I need to know in all honesty if Ciara or the Jestream issue means I’m in for a rough ride, as I really don’t want this flight to ruin the family hols later in the year by causing a refusal.

I know we've been here (many times) before, but short-haul holidays are still available.


 
Posted : 06/02/2020 8:36 am
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Take your mind off it by trying to master the Baltimore accent before you set off (a little sweary):


 
Posted : 06/02/2020 8:58 am
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Also Storm Ciara probably won't be as severe as predicted when  it finally arrives late on Saturday night.

The Express, Torygraph & Daily Fail spend an awful lot of time creating headlines designed to scare old people into an early grave. They then sit back in an almost post-coital pleasure as they see mortality rates surge. Utter twunts.

I have to go back to the US soon for work and I'm dreading it but in reality is the utter boredom and jetlag which is the true enemy, not the aerodynamics.

Try to visualise your trip, try calming tricks and tips, stay off the booze on the plane, stretch, stay hydrated and talk to the aircrew of you need reassurance.


 
Posted : 06/02/2020 8:58 am
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Thanks for the helpful comments.   Its a work trip, I wouldn't consider Detroit-on-Sea in February for a holiday.


 
Posted : 06/02/2020 9:09 am
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Yes, but in your first post you’re already talking about the inevitable Caribbean trip later in the year. Given how far off that is, it’s not good to already be starting to stress


 
Posted : 06/02/2020 9:13 am
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Detroit-on-Sea in February for a holiday.

I thought you were going to Baltimore? Are you doing a further flight to Detroit?


 
Posted : 06/02/2020 9:25 am
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The Aquarium is brillant.

Otherwise its just like the Wire.

You'll be absolutely fine. The margin of safety they employ is huge.


 
Posted : 06/02/2020 9:41 am
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inevitable Caribbean trip

I'm not going there and didn't mentioned the Caribbean.  I'm leaving now as the last thing I need is an internet argument


 
Posted : 06/02/2020 9:47 am
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There is no argument, you said "I really don’t want this flight to ruin the family hols later in the year by causing a refusal" so it's clearly already playing on your mind.


 
Posted : 06/02/2020 9:50 am
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I don't know if it will help and I'm probably stating the blindingly obvious but,

Can you rationalise it, what is turbulence? You say you're worried about a "rough ride" but what does that really mean?

The reality is if you hit the worst turbulence you've ever known, the plane will jostle about for a few seconds. That's it. In a worst-case scenario you might spill a bit of your orange juice. Planes falling out of the sky because it's a bit windy just doesn't happen, we've been doing this for a hundred years and we've got quite good at it now.

Phobias aren't rational I know and I certainly don't mean to trivialise the fear (I have a couple myself and they're stupid) but maybe this might help override the irrational side of it a little?

The Express, Torygraph & Daily Fail spend an awful lot of time creating headlines designed to scare old people into an early grave.

The Express in particular is notorious for making up weather reports. Every summer will be the hottest on record, every winter the coldest, they've been doing it for years.


 
Posted : 06/02/2020 3:02 pm
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... or failing that, go see your doctor and take half a Valium before you get on board. My ex was in the same position as you and successfully got a prescription for like two pills rather than a large pack of them.


 
Posted : 06/02/2020 3:05 pm
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It’s colleagues of mine that plan which paths the airlines take across the Atlantic. Google NAT tracks and take a look at the Jeppeson charts that show the specific tracks relative to the jet stream. For Baltimore I would expect your flight to route south of the forecast jet stream, but it’s not unheard of to route north over the east coast of Scotland before heading over Greenland and back down the east coast of the U.S. Airlines, particularly U.S carriers are very careful about avoiding even the lightest turbulence.
More often than not, it’s crossing the jet stream at 90 degrees to the flow that causes mod-severe turbulence. Looking at the forecast it doesn’t look like that’s going to happen.

Edit: NAT forecast


 
Posted : 06/02/2020 3:11 pm
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Second getting some drugs. They chill you right out.


 
Posted : 06/02/2020 3:14 pm
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Are you driving to the Airport? Many times more dangerous than Flying.


 
Posted : 06/02/2020 4:26 pm
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You have more issues when you land. Baltimore. For work. I hope they are payng you danger money.

It's generally not advisable to stick your arms in the air and go "weeeee" during turbulence. It seems to unsettle the other passengers. So i've been told.


 
Posted : 06/02/2020 6:25 pm
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You have more issues when you land. Baltimore. For work. I hope they are payng you danger money.

+1

Flying's the safe bit. Baltimore is quite 'tasty'...


 
Posted : 06/02/2020 6:58 pm
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Kryton - the Jetstream is always there - it’s just the fact that it moves up and down a bit that changes.

The strong Jestream winds over the Atlantic tend to flow from West to East. To avoid the strong headwinds and save fuel when heading to the US, we are normally planned to avoid the jetstream, whereas flying home we aim for the strongest tailwinds.

One of my best flights home from Chicago was right in the middle of a 180mph jetstream carrying me home super fast! Not a bump in sight.

Basically - it will be no different to any other flight.


 
Posted : 06/02/2020 9:22 pm
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@dantsw13 - have you been on a bombardier cs300?

Do you know why the engines sound like they are screaming? Swiss use them on the Manchester-Zurich route and they're really annoying. Heading back out there next month again..🙉🙉🙉


 
Posted : 06/02/2020 9:40 pm
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… or failing that, go see your doctor and take half a Valium before you get on board. My ex was in the same position as you and successfully got a prescription for like two pills rather than a large pack of them.

I’m imagining OP off his face on pills and free spirits re-enacting ‘The wolf of Wall St’ Lamborghini scene when he gets over to the other side 🙂


 
Posted : 06/02/2020 10:15 pm
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Elshalimo - never been on the C series (now known as the Airbus220) but new modern engines do tend to make a more whiney noise.


 
Posted : 07/02/2020 9:35 am
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@dantsw13 - thanks.
They scream like a banshee when they're climbing to cruise altitude and it takes a long time for them to calm down whilst cruising. I was really worried the first time I flew on one!!
I'm a nervous flyer and they also seem very manoeuvrable, or maybe the Swiss pilots just like throwing them around?


 
Posted : 07/02/2020 10:20 am
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The bombardier utilises P&W's latest geared turbofan engines. Engine noise comes from a number of sources - buzzsaw from the fan, usualy that raspy sound you hear during initial climb when fan speed is high and air density is also high and the blade tips are going transonic, turbine noise and bleed valve noise: at low altitudes the compressors are pulling in too much air so some of it needs to be dumped overboard so bleed valves are open during take off and most of the climb.

In order to achieve the significant strides in fuel efficiency engines have in the last 20 years they have become much higher in their bypass ratio - that is why the fan diameter has increased so much over the years. The larger the fan the slower the turbine (speed limited to fan blade tip speed), the lower the turbine noise. However if you slow down the turbine too much as you grow the fan size you loose turbine efficiency and negate the fuel efficiency benefit of the higher bypass ratio and larger diameter fan.

So the new P&W geared fan features a reduction gearbox enabling the turbine to spin much faster while the fan can turn much more slowly. A good portion of the turbine whine noise will be the faster spinning turbine and probably some bleed valve noise as the faster turbine will require more air to be dumped from the compressor at take off and climb. Remember the old 737-200's with their turbojet engines? they whined like a banshee as they were turning much faster than modern day high bypass ratio engines. In the strive for better fuel efficiency we're returning to faster spinning turbines.

The screaming sound is good...turbines like to scream. Its when they don't scream you have to worry!


 
Posted : 07/02/2020 10:38 am
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I do wonder whether it’s the noise muffling technology, to reduce noise footprints around airports, leaves behind that high pitch.


 
Posted : 07/02/2020 10:41 am
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No muffling technology. Muffles are heavy, draggy and sap efficiency. Primary engine noise reduction is done by optimisation of the compressor and turbine....remember the old WW2 air raid sirens? That is an example of the opposite of design we're trying to achieve with gas turbine engines - so the number of rotating turbine and compressor blades vs.the static turbine and compressor vanes impacts noise massively due to the interactions of the gas flow within the engine, also the shape of the ducting within the core and a lot around combustor design. The rest is around the nacelle design to shield noise utilising noise suppression materials and ensuring the nacelle covers areas where turbine and compressor noise can escape - like the bleed valves. That is why 777's with GE engines and 787's with GEnx engines make such a racket on start up - they have very long engine cores and the nacelle doesn't do such a good job of covering up the bleed valve noise. You can't extend the nacelle as that adds weight and drag...so its the classic engineering quandary of balancing competing attributes - noise, fuel efficiency, weight etc.

Ultimately though engines are much quieter than they used to be they're always going to generate some noise. And though there are strict regulations around how much noise the engines generate externally there are none to regulate the noise you hear in the cabin. You can have an engine installation that is pretty quiet externally, but noisy as hell in the cabin - thought a lot of this has to do with the design of the fuselage of the aircraft and the attention to cabin noise the airframe makers invest.

Ultimately though apart from take off the majority of the noise from an aircraft is from the airframe and not the engine. Flaps and undercarridge makes a lot more noise than engines on landing.


 
Posted : 07/02/2020 10:58 am
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Wobbli - you’re obviously far more technically minded than me!!!!


 
Posted : 07/02/2020 11:06 am
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And this ladies and gents is why we love STW so much. sometimes.

Although the pilot just confessed to not being technical (mild exaggeration) 😉


 
Posted : 07/02/2020 11:14 am
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By the way, the CS300/A220 is a nice plane to fly on in economy.
Roomy, comfy and Swiss always give you free chocolate!!


 
Posted : 07/02/2020 12:01 pm
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Wobbli – you’re obviously far more technically minded than me!!!!

Not at all, it's just my job and I have access to the real clever bods who design them. I talk to pilots and airlines alot time about engine technology and how they can operate engines to be more mechanically sympathetic and eek out more fuel performance for their airline accountants. The real geniuses are the guys and girls actually designing this stuff and their intention is to make them invisible to the pilot - all you want is thrust when you demand it and that's what we're trying to achieve. It really is on the edge of physics and our technical understanding. A gas turbine is a classic example of something that really is such a simple thing that has been made extremely complicated in the eternal pursuit of efficiency. I'm just a failed engineer who loves to talk about engines.


 
Posted : 07/02/2020 12:37 pm
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Hopefully Kryton has a good flight today .


 
Posted : 08/02/2020 8:59 am
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*Like*


 
Posted : 08/02/2020 9:41 am
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I questioned a united airlines that was going to San Fran from Heathrow. The reason being his flight plan was very unusual because he was flying out over East Anglia towards norway. Normally it's via Scotland.

I'm paraphrasing but his reply was, "some smart ass in flight planning filed the route for us. We doubled checked it but they're right. Because of the winds they've saved us 1.5hrs!"

Track miles it was longer to go via Norway and the pole but there was a hoofing southerly wind up over norway.

Anyway, I'll put my anorak back on.


 
Posted : 08/02/2020 10:14 am
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Just to clarify regarding engines:

In the Trent 700s on my current steed (and like their 1970s predecessor, the magnificent RB211s on a previous steed) we have acoustic lining in the intakes to reduce the noise of the compressors. Acoustic lining is present in most, if not all modern airliner intakes. It looks like the stuff in the old style phone booths with lots of holes in it. It deadens some of the high pitched sounds coming from the compressor.

Pretty much all engines will have their 1st stage RPM limited by fan tip speed (as do turboprops) to avoid any supersonic effects. Part of an intake's function is to reduce airflow rates so the engine is always using subsonic air (think concorde intake ramps). Small engines will have similar tip speed as to, say the Trent, which is large, but the fact the RPM is so much higher will result in higher frequencies, hence the higher whine. There's no getting round that one.

Regarding airflow and bleeds to dump any excess: Any "excess" simply goes down the bypass and shields the exhaust gases from the core, reducing the shear noise, the roar that all jets make. They do tend to have variable inlet guide vanes (IGV) which control the amount of air going into the core, but there are no 'bleed valves' as such in normal (non-starting) operation (as there can be bleed valves used there), other than compressor bleeds but those are for pneumatic systems rather than flow control.

Fighters sometimes have flaps near the intake; these open to at slow speed to allow higher mass flow rates, but close at normal speeds, but these don't 'bleed' air out.

The latest Trents have a jagged rear on the bypass duct; this induces turbulent airflow, reducing the shear experienced by the air and therefore noise.
It's been a very very long time since airliners have been using Turbojet engines; they were low bypass turbofans in the early days (727s) and noisy, and became higher bypass as technology improved (L-1011s).

Don't start me on Geared Turbofans! Nothing wrong with a Triple Spool High Bypass engine!


 
Posted : 08/02/2020 11:38 am
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Well the original question was around why the P&W GTF on the Bombardier was so noisy. Whilst the bleed valves dump excess air into the fan duct that air is very noisy - its very high pressure being squirted through a valve, and when the bleed valve is open you also get alot of noise from the actual compressor leaking out which is hard to suppress. The engine nacelle does suppress most of this noise but some are better than others at it. The RR engines are very short in length due to their 3-shaft architecture so the bleed valves are nestled well within the engine nacelle so do a good job of suppressing noise. Engines with 2-shaft architectures are much longer and the bleed valves tend to be positioned closer to the exit of the fan duct therefore the noise suppression isn't as good. The P&W GTF has a faster spinning core which will generate more noise to begin with anyway so the engine nacelle has a tougher job dealing with that so why that engine will probably sound like it does. Never flown on one so can't comment. But I'm not sure how good the Boeing 787 Chevron system is (it's a Boeing design not an RR or GE design). They don't have it on the 777X and didn't have it on their proposed NMA and the 737MAX doesn't have it either, so either it is a design that has to be considered from day 1 along with the whole system to get the benefits or it isn't very effective at all. The old RB211-524 had the pastry cutter on the core exhaust which attempts to do the same thing as the Boeing chevrons. Effective but at a sizable weight penalty.

But I'm a 'Trent man' too and triple spool is the business....but if us in the aerospace industry truly want a sustainable future we have to strive to continue and accelerate the trend from the past 50 years in continual fuel efficiency improvements, and the only way we're going to do that is to increase fan diameter. On the latest big Trents the LP Turbine (which drives the fan) is already getting so big and heavy it's getting hard for the airframers to physically design wings and pylons to take them. If we want to reduce the size and efficiency of the turbines while increasing the fan size and bypass ratio then unfortunately even the 3-shaft architecture has reached the end of the road. We're at bypass ratio's of 10:1 on the latest engines (over double that of the Trent 700 engines) and we need to be heading to bypass ratio's of 15:1 or more for the next generation of engine to get the efficiencies we need. On the big wide body engines that means fan diameters of around 135 inches vs the circa 97 inch of the mighty Trent 700.


 
Posted : 08/02/2020 2:13 pm
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Hopefully Kryton checks back in to this thread before his flight - should certainly help him sleep!! 😎


 
Posted : 08/02/2020 2:20 pm
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Hopefully Kryton checks back in to this thread before his flight – should certainly help him sleep!! 😎

Are you saying this isn’t riveting??!


 
Posted : 08/02/2020 6:32 pm
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Well...........


 
Posted : 08/02/2020 8:02 pm
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Hopefully wandering around downtown with an Oriels hat on.


 
Posted : 08/02/2020 10:07 pm
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It started well.  I got a CBT session Thursday night which bolstered my confidence then checked in Friday afternoon to get a free upgrade to a Premium bulkhead seat...

...then woke at 3am Saturday ejecting fluids from every orifice and haven’t eaten since.  Currently at home with a fever and even my skin & hair hurts. Will be seeing a Dr tomorrow.


 
Posted : 09/02/2020 9:00 am
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Flights have been setting speed records US-UK

https://www.independent.co.uk/travel/storm-ciara-flight-record-new-york-london-time-atlantic-winds-british-airways-a9325396.html

Feel for you Kryton, anxiety is a ****


 
Posted : 09/02/2020 10:13 am
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Sorry to hear that Kryton. Get well soon.


 
Posted : 09/02/2020 10:19 am
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@Kryton57

I think you need to change the thread title to Gastroenterologists of STW - help please!!

Speedy recovery...


 
Posted : 10/02/2020 10:07 am
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Wobbliscott is absolutely right in what he says. There are multiple bleed valves besides those used to tap off IP and HP air for anti-ice and air conditioning (if not on a 787).

The Trent 1000 has at least 10. Rolls Royce state that "during engine operation certain bleed valves will open or close at pre-determined shaft speeds to assist engine handling [and] control the volume of air flowing through the engine compressor systems".


 
Posted : 10/02/2020 10:33 am
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I think you need to change the thread title to Gastroenterologists of STW – help please!!

I’m not one to go off sick easily but still unable to keep food down, I went to the docs today and got diagnosed with viral gastro, a mild case of flu on top and fatigue.  Signed me off for a week and told me to rest and spend the next 2 days on water only.

Incredibly bad timing for the trip as there’s some important stuff going on at work and all I can do is sit around bored near the bathroom with my headache and hurty skin.

Anyway, I quite like the reading about engines in the short bursts I can manage, thumbs up and thanks for the get wells.


 
Posted : 10/02/2020 5:45 pm
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Flaperon

There are multiple bleed valves besides those used to tap off IP and HP air for anti-ice and air conditioning

I didn’t say otherwise, however I accept I don’t know about the P&W setup, and indeed there are bleed tap-offs on the 700 to manage core flow for stable operation..... I stand corrected (I thought it was the function of the IGVs)!


 
Posted : 10/02/2020 8:01 pm
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You're both right, it is indeed the primary function of the variable inlet guide vanes (VIGV's) to manage the compressor handling, but at low altitudes where the air is dense bleed valves are needed on all gas turbine engines to prevent compressor stall and surge. Engines are not optimised or sized for their maximum thrust at take off...they're optimised for cruise as that is where they spend most of their working life. So at take off you're just working the machine harder and you can't build a machine that is optimised for two points of its operating envelope so you have to make compromises and bleed valves are just that.

The RB211-535 (which powered the 757) actually dispensed with VIGV's altogether and managed compressor handling purely from bleed valves (well it was actually a circular band) and did away with the complexity of the VIGV system altogether at the expense of a bit of efficiency - you ideally don't want to dump air from the compressors as you've invested expensive fuel to compress the air only to dump it overboard without earning you any revenue. But complexity and reliability are important attributes as well. No good having a super efficient engine if it's always going tech and causing delays and cancellations. This was one of the secrets of the 535's unbelievable reliability back in its day, which is still impressive today and was orders of magnitude better than engines of its generation.

But modern engines use pressurised fuel to actuate the VIGV mechanism and electronic control systems to manage control, which is far more reliable than the old and problematic pneumatic systems of previous generations of engine like the 535, so VIGV system reliability is no longer a problem.

But yes, though the bleed valves are only open for take off and part of the climb phase air is being siphoned from the compressors all the time for various things - anti ice, turbine cooling, turbine blade clearance control, engine sealing, cabin air (i.e. the air you breath comes off the engine compressors)...and the designers are always looking for ways to make the air siphoned off work harder to minimise it's use and eek out more and more efficiency from the engine - continually chasing fractions of a percent of efficiency.

Kryton - sorry to hear about your bought of illness. I hope it wasn't brought on by concerns about your up and coming flight.


 
Posted : 10/02/2020 8:59 pm
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It's all getting very techy on the engine front. As Kryton is spending some time on the throne, my contribution to this thread is to point out that when you flush the toilet, it's the low pressure outside that sucks the contents out into the tank. Apparently when the aircraft is on the ground (and also presumably at low altitude) a pump is used to create a vacuum in the tank. I was going to say compressor, but what's the opposite of a compressor?


 
Posted : 10/02/2020 9:08 pm
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Expressor?


 
Posted : 10/02/2020 9:51 pm
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On the basis the fundamental workings of a jet engine are suck, squeeze, bang, blow (though technically not correct but a close enough approximation), the compressor part is squeeze, so the opposite of the compressor bit is the blow bit, which is the turbine. Which is fitting as that part on the throne can be very hot too, especially after a good Jalfrezi.


 
Posted : 10/02/2020 10:08 pm
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Please lord, won't anybody think of Kryton? 🙁


 
Posted : 11/02/2020 12:27 am