Airspeed guesstimate for you polity types
Discussion
Bonjour from Mauritius! So, on the way back last night from Melbourne, we apparently had a monstrous head wind.....the journey there was just over 9 hours but it took nearly 12.5 to get back....I took a pic of the inboard display and it shows a ground speed of 659km/h (408mph) at 10475 metres. Now what sort of head wind are we looking at and would this be a fairly low cruising speed
for that height without a head wind? Aircraft is an A340E with, I guess, pretty much only half a load of passengers.
for that height without a head wind? Aircraft is an A340E with, I guess, pretty much only half a load of passengers.
It would appear that you were flying directly in to the jet stream and that has a wind speed of anywhere from 100-200mph depending on height etc.
http://en.wikipedia.org/wiki/Jet_stream
http://en.wikipedia.org/wiki/Jet_stream
TVR1 said:
Bonjour from Mauritius! So, on the way back last night from Melbourne, we apparently had a monstrous head wind.....the journey there was just over 9 hours but it took nearly 12.5 to get back....I took a pic of the inboard display and it shows a ground speed of 659km/h (408mph) at 10475 metres. Now what sort of head wind are we looking at and would this be a fairly low cruising speed
for that height without a head wind? Aircraft is an A340E with, I guess, pretty much only half a load of passengers.
I'd imagine in a 340, at 35,000ft on a typically economic cruise setting, your true airspeed would be in the region of 465kts. 408mph is about 355kts, so you were facing a headwind of circa 110kts. Pretty substantial, but weather patterns in SE Asia are very interesting/obscure..for that height without a head wind? Aircraft is an A340E with, I guess, pretty much only half a load of passengers.
Simpo Two said:
Another reason to allow extra fuel (see 'avoiding storm' thread)
Indeed. Flight planned fuel is predicated on forecast weather conditions, & flight planning tools (at least at my company) are generally very accurate. Occasionally, however, you do raise an eyebrow when looking at the flight plan calculations and the raw weather data!Always pays to check...
110 knots looks about right. You can play animated charts of the Jet Stream (at ~30000ft) in the Southern Hemisphere over the past few days at http://squall.sfsu.edu/scripts/shemjetstream_model...
ETOPS said:
Indeed. Flight planned fuel is predicated on forecast weather conditions, & flight planning tools (at least at my company) are generally very accurate. Occasionally, however, you do raise an eyebrow when looking at the flight plan calculations and the raw weather data!
So if a flight was delayed by, say, 12 hours, and in that time the weather changed from 'no headwind' to 'strong headwind', the bowser would have to nip out and put a splash more in the tanks?And - as fuel is measured by weight, how is it weighed/quantified once in the 'plane?
Simpo Two said:
And - as fuel is measured by weight, how is it weighed/quantified once in the 'plane?
Well you know how much you've ordered and that is cross checked as it is delivered. Adjustments are also made for temperature (of the fuel). Once it's on board the burn rate of each engine is depicted and you cross check this against the quantities shown in the tanks.Typically we'll calculate our requirement in lbs, order in litres and monitor the quantity and burn (fuel flow) in lbs. That's for europe, the old eastern bloc and asia. In the US it's sold in US gallons so a conversion chart and calculator are always handy.
Simpo Two said:
So if a flight was delayed by, say, 12 hours, and in that time the weather changed from 'no headwind' to 'strong headwind', the bowser would have to nip out and put a splash more in the tanks?
And - as fuel is measured by weight, how is it weighed/quantified once in the 'plane?
Yes, exactly. Or offload baggage, if your tanks are already full. And - as fuel is measured by weight, how is it weighed/quantified once in the 'plane?
Typically on a flight plan, there are many subcategories of fuel. There is trip fuel - which is the fuel needed to go to your destination, with the forecast weather. There's alternate fuel - which takes you from your destination to the alternate. There's reserves, which you have to land with (or declare a fuel emergency), there's mandatory fuel - which is extra fuel which drags you to somewhere safe in the event of a decompression (and subsequently having to fly for hours at a low altitude).
Then there are the two categories which may be of interest during developing weather:
Contingeny - which is a fuel buffer, for example - making a dogleg around weather, or being held at a lower altitude by ATC.
Rec. extra - the fuel the Captain decides to take, in addition to the above. Eg - expecting poor weather at the destination, a little extra will allow me to hold there if ATC gets backed up - because when we get down to our alternate fuel + reserves, one must divert. That said, it costs a lot of money to carry extra fuel, and it's very rare that a diversion happens. Where I work, they're happy to carry the can of the cost of diverting, as it is cheaper than every aircraft lugging a couple of thousand kilos around with them..
The specific gravity of the fuel is checked, and it is ordered in weight (not volume), and it is quantified on the flight deck as a weight, and it's consumption is measured in weight.
Thanks ETOPS, very interesting.
ETOPS said:
The specific gravity of the fuel is checked, and it is ordered in weight (not volume), and it is quantified on the flight deck as a weight, and it's consumption is measured in weight.
Indeed, but how do they actually weigh it in the tank - you can't have a set of scales at the bottom of the tank and you can't weigh the whole 'plane and subtract passengers + baggage as it would be too variable...Simpo Two said:
Thanks ETOPS, very interesting.
Umm, they know the volume that is pumped in when it's filled (and thus the weight as well) and the fuel flow to the engines is measured very accurately so they know the volume (and thus weight) being used. I expect that these measures are backed up by in-tank sensors to get an actual reading...ETOPS said:
The specific gravity of the fuel is checked, and it is ordered in weight (not volume), and it is quantified on the flight deck as a weight, and it's consumption is measured in weight.
Indeed, but how do they actually weigh it in the tank - you can't have a set of scales at the bottom of the tank and you can't weigh the whole 'plane and subtract passengers + baggage as it would be too variable...You don't weigh it in the tanks at all, it's a known amount based on Specific Gravity. You simply select fuel amount in KG or tonnes on the aircraft's refuelling panel and the bowser gives you that amount and then it cuts off automatically.
All the paperwork is based on weight not volume.
All the paperwork is based on weight not volume.
TVR1 said:
Bonjour from Mauritius! So, on the way back last night from Melbourne, we apparently had a monstrous head wind.....the journey there was just over 9 hours but it took nearly 12.5 to get back....I took a pic of the inboard display and it shows a ground speed of 659km/h (408mph) at 10475 metres. Now what sort of head wind are we looking at and would this be a fairly low cruising speed
for that height without a head wind? Aircraft is an A340E with, I guess, pretty much only half a load of passengers.
I wasn't on that night, think I was on a few nights before with the Perth-Mauritius flight so one of my colleagues was on last night. That night the Perth Mauritius flight was an A332 which has to use ETOPS type tracks to get over to Mauritius. They were doing about 370kts groundspeed at one point on the night I was on. for that height without a head wind? Aircraft is an A340E with, I guess, pretty much only half a load of passengers.
The Mauritius planes will regularly take blocks of levels up to avoid turbulence and headwinds and get the "optimum cruise level" for that time of the flight.
The A343 from Melbourne to Mauritius generally uses the same track all the time so they normally end up straight into the jetstream. For example, there are 150kt westerlies across the Great Australian Bight tonight.
Edited by motomk on Friday 3rd June 10:17
motomk said:
TVR1 said:
Bonjour from Mauritius! So, on the way back last night from Melbourne, we apparently had a monstrous head wind.....the journey there was just over 9 hours but it took nearly 12.5 to get back....I took a pic of the inboard display and it shows a ground speed of 659km/h (408mph) at 10475 metres. Now what sort of head wind are we looking at and would this be a fairly low cruising speed
for that height without a head wind? Aircraft is an A340E with, I guess, pretty much only half a load of passengers.
I wasn't on that night, think I was on a few nights before with the Perth-Mauritius flight so one of my colleagues was on last night. That night the Perth Mauritius flight was an A332 which has to use ETOPS type tracks to get over to Mauritius. They were doing about 370kts groundspeed at one point on the night I was on. for that height without a head wind? Aircraft is an A340E with, I guess, pretty much only half a load of passengers.
The Mauritius planes will regularly take blocks of levels up to avoid turbulence and headwinds and get the "optimum cruise level" for that time of the flight.
The A343 from Melbourne to Mauritius generally uses the same track all the time so they normally end up straight into the jetstream. For example, there are 150kt westerlies across the Great Australian Bight tonight.
Edited by motomk on Friday 3rd June 10:17
t, we arent going to make it?' and then what do they do? turn around? or is there anywhere in the Indian Ocean that they can put down?TVR1 said:
Cheers motomk...must ne what we ran into. One question though, obviously weather is predicted and there are safety margins on fuel and I understand what the ETOPS is all about.....but what happens if there is a totally unexpected and long lasting fierce headwind? At what point do the pilots think 'st, we arent going to make it?' and then what do they do? turn around? or is there anywhere in the Indian Ocean that they can put down?
Critical Points (CP) and Equi-Time Points (ETP) are calculated in advance and appear on the plog. On reaching these you know how you are progressing and if you need to divert where to go, how long it will take and the fuel burn getting there. Constant cross-checks tell you how much fuel you have at enroute waypoints compared to what you planned for. Minor differences are fine but a worrying trend ie, lower groundspeed will give early warning. t, we arent going to make it?' and then what do they do? turn around? or is there anywhere in the Indian Ocean that they can put down?Done properly there's no chance of having an 'oh s
t' moment out over the wet in the dark.
el stovey said:
You don't weigh it in the tanks at all, it's a known amount based on Specific Gravity. You simply select fuel amount in KG or tonnes on the aircraft's refuelling panel and the bowser gives you that amount and then it cuts off automatically.
All the paperwork is based on weight not volume.
Indeed, since the volume changes with temperature. Got that.All the paperwork is based on weight not volume.
Soooo.... to find the weight they first measure the volume, then use the temperature to convert it to weight... right.
Then you have to hope that the meters that measure flow to the engines are totally accurate or you're going to get a cumulative error and then Newton wins.
Didn't a 'plane crash once because they got litres and gallons mixed up? I'm surprised it doesn't happen more often.
Edited by Simpo Two on Friday 3rd June 14:45
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