Ask The Pilot

[harvyk]

Is there a reason that they only use the 744's for the Antarctica charter flights?

 

[jb747]

PER if before point of no return?
RUN if after point of no return?

You can get all the way across using Perth, Reunion, Durban, Joburg. You're about 1800 NM from an airport in the worst case. Less than some ETOPs.

But point of no return is further west along the southern ocean due to prevailing winds?

Use a different term. Last point of diversion. The return is simply a diversion back to the start point. There's alway one airport which is the closest. The aim is not to have a gap between the last point for one place, and the first for the next.

Legally, you must always have somewhere to go engine out, and separately, depressurised. They may not be the same airport, though in the Indian Ocean they most likely are. The two cases are not considered together...if they were most long distance flights simply could not happen. It's very common to have to carry extra fuel for the depressurised case.

Wind is considered, and yes, PNR is further away if you will have a tail wind on return.

 

[jb747]

Is there a reason that they only use the 744's for the Antarctica charter flights?

'Cos it has four of everything, and isn't affected by ETOPs. And the company can generally find one to use....

 

[Quickstatus]

Legally, you must always have somewhere to go engine out, and separately, depressurised. They may not be the same airport, though in the Indian Ocean they most likely are. The two cases are not considered together...if they were most long distance flights simply could not happen. It's very common to have to carry extra fuel for the depressurised case.

Meaning the risk of a combined engine out and depressurisation is deemed to be so low that planning does not consider both together?

 

[jb747]

Meaning the risk of an engine out and depressurisation is deemed to be so low that planning does not consider both together?

Exactly, though it's easy enough to come up with scenarios which would give you both. All of the rules, and especially ETOPs, are a form of gambling. The odds are very low, but ....

 

[Quickstatus]

Re crew meals:

I thought the crew are offered similar meals to the J meals?. If given the option would you prefer the passengers offerings?

 

[jb747]

Re crew meals:

I thought the crew are offered similar meals to the J meals?. If given the option would you prefer the passengers offerings?

The crew meals are totally separate to any passenger meals. They're about on par with economy...certainly not J. Sometimes better is offered.

 

[flyer89]

The good old PNR. Covering all of this at the moment in flight planning. Except with the ultra modern 727.....

 

[jb747]

The good old PNR. Covering all of this at the moment in flight planning. Except with the ultra modern 727.....

I think when I did the subjects it was a mythical aircraft that seemed very like a DC8. It really doesn't matter what the aircraft is, the theory is the same.

Now, can you work out the diversion point, allowing for wind, using a piece of thread, a map, and a couple of pins?

 

[flyer89]

Now, can you work out the diversion point, allowing for wind, using a piece of thread, a map, and a couple of pins

Well I am a millennial after all.....so no.

Just eyeballing some of the fuel burn figures in comparison to today’s aircraft it was such an inefficient and uneconomical old girl. Which for some reason I have a growing affection for.

 

[RooFlyer]

Now, can you work out the diversion point, allowing for wind, using a piece of thread, a map, and a couple of pins?

I'm pretty sure the answer to that is amongst the 12,000 or so previous posts. Give me a moment ....

 

[Quickstatus]

Now, can you work out the diversion point, allowing for wind, using a piece of thread, a map, and a couple of pins?

Yes we did this exercise last year:
Ask The Pilot.

 

[jb747]

In all of these pages I'm sure I've repeated myself, and probably contradicted myself too.

Nevertheless, for someone learning the concept, it's a rather neat way of seeing the problem. And in the real world, you take a guess, plug it into the FMC, and see what answer it gives. There's still a small level of pride in being able to solve it without an FMC...though I'm more likely to have one of them, than the required map, chart, and pins.

 

[flyer89]

Always appreciated. As an observer of this thread for many years it has often taught me a lot more than pprune sadly.....

 

[AviatorInsight]

Always appreciated. As an observer of this thread for many years it has often taught me a lot more than pprune sadly.....

And dare I say, it will continue to do so?

 

[AviatorInsight]

So a 737 could with usual passenger/freight loads could legally do BNE-PER-ADL on one fill ?

Doing the reverse last night, fuel was as follows:
158 pax to PER
170 pax to BNE
0.2T taxi
8.6T ADL-PER
11.2T PER-BNE
Total flight fuel = 19.8T. Given the capacity is 20.5T we’d be landing with no fixed reserve, and about 15mins fuel, the most tailwind we saw was a measly 70kts which was 70kts of headwind going to PER.

So to answer the question, could it legally be done? No and even still it’d be an overfly of PER.

 

[Quickstatus]

How do the combustion chambers in a jet engine stay lit when airflow through engine is hundreds of knots

 

[jb747]

How do the combustion chambers in a jet engine stay lit when airflow through engine is hundreds of knots

The answer lies in the fact that the airflow is actually much, much slower than that.

More than you ever wanted to know...here
http://www.ijceronline.com/papers/Vol3_issue12/Version-2/G031202036040.pdf

and here
Why do gases in the combustion chamber only flow one direction to the gas turbine in a jet engine?

 

[Quickstatus]

Thanks. I get it I think.

I did not understand how the compressor stages by increasing pressure could also lower velocity for combustion

The compressor stages have fans AND static vanes - fan vane fan vane. This has the effect of increasing pressure and reducing velocity. ... I think.

And the turbine stages have static vanes but t in reverse order - vane fan vane fan

Can you please explain the startup sequence in a cold turbofan

Also:

Diesel cars have high pressure injectors to create small diameter fuel droplet for efficient combustion. Same in jet combustors?.

 

[jb747]

I did not understand how the compressor stages by increasing pressure could also lower velocity for combustion.

The compressor stages have fans AND static vanes - fan vane fan vane. This has the effect of increasing pressure and reducing velocity. ... I think.

And the turbine stages have static vanes but t in reverse order - vane fan vane fan

As far as I can recall, all the vanes are doing is setting the flow up for the next rotating stage i.e. controlling and directing the flow.

Can you please explain the startup sequence in a cold turbofan.

To be cheeky...I select start, then place a fuel control switch to run. The entire start, including most aborts, is automatic.

Basically, you use an air source to drive a starter that winds the engine core up to about 15-25%. The higher the better. The fan may not be rotating at that point. Fuel and ignition is then introduced, and the core will start to accelerate. The temperature rises very rapidly during this stage. In a normal start the temperature will peak and then wind back to about 60% of that value as the stages accelerate, and normal air flow starts. The starter disengages at about 45-50%. If it doesn't disengage, the start will need to be aborted, as the starter will overspeed. Ignition cuts out at about that point.