Ask The Pilot

[harvyk]

Is a descent from 38,000ft to 9,800ft in around 8 minutes normal? The level off at 9,800 makes me think pressurisation problem.

https://flightaware.com/live/flight/QFA825/history/20160930/0400Z/YPDN/YPDN/tracklog

Qantas plane makes emergency landing at Darwin International Airport - ABC News (Australian Broadcasting Corporation)

(runs and hides as I'm not a pilot)

 

[jb747]

Is a descent from 38,000ft to 9,800ft in around 8 minutes normal? The level off at 9,800 makes me think pressurisation problem.

https://flightaware.com/live/flight/QFA825/history/20160930/0400Z/YPDN/YPDN/tracklog

Looks like that to me too. It doesn't actually mean it depressurised though. If there are pack issues it is possible to get the aircraft down before the rubber jungle is tripped.

Looking at the article, there's no mention of the masks, so I'd guess that for whatever reason they ended up without packs, in which case the cabin climbs relatively slowly (3-500 fpm). If you don't waste any time getting down, the cabin will still be low enough for the oxygen system to remain 'inert'.

 

[Captain Halliday]

Looks like that to me too. It doesn't actually mean it depressurised though. If there are pack issues it is possible to get the aircraft down before the rubber jungle is tripped.

I checked with airport connections whom I consider reliable. I'm told the masks were not deployed.

 

[Quickstatus]

Looks like that to me too'.

JB747, it appears there was a slight positive change in velocity when the altitude change occurred but then during the main part of the descent there was a negative change in velocity (deceleration).

A mere layman would surmise that an altitude change such as this would involve a pitch the nose down and push the throttles forward type of manoeuvre to get down ASAP.

FA suggests otherwise - that the descent was associated with a slowing of the aircraft.

How is the energy of the aircraft reduced?

 

[jb747]

JB747, it appears there was a slight positive change in velocity when the altitude change occurred but then during the main part of the descent there was a negative change in velocity (deceleration).

Aircraft are flown using IAS and mach. Mach is used at high levels, and basically defines the maximum speed, whilst IAS is what makes the wings work.

As you climb, the air becomes thinner, so if you were to climb at a 'real' 300 knots (TAS) the IAS would be reducing. We do it the other way, and climb at a constant IAS, so the TAS increases as we climb (at 40,000' TAS is approximately double the IAS).

In a descent, if maintaining a constant IAS (as you would), the TAS will be reducing. The speeds shown on the web site referenced are ground speed, so that's the TAS adjusted for wind.

A mere layman would surmise that an altitude change such as this would involve a pitch the nose down and push the throttles forward type of manoeuvre to get down ASAP.

This is a theory that I've never understood. Take your car to the top of a hill. Point it downhill, and give it a boot full of throttle. What will happen?

A descent involves the conversion of potential energy into kinetic, with drag having the effect of wasting a portion of the energy. Any time you add power, you add energy to the mix...which you don't want. A normal descent involves a pitch change of 2-3º, and reduction of power to idle.

If you want a more rapid descent, you can increase the level of energy converted to drag by one of two methods...
1. Increase the IAS. Drag is proportional to the square of IAS, so that will increase the rate of energy loss. But, you don't increase the speed by adding power...you do so by lowering the nose a degree or two, letting it accelerate, and then raising the nose slightly to hold that speed. Speed is being controlled by the pitch attitude.
2. Use the speed brakes. They increase the drag, and will require the nose to be lowered another degree or two to hold the speed.

This theory about the use of power in descents may come from a misunderstanding of the power increases that are felt during a normal arrival. In that case, we are generally trying to fly a vertical profile, so any time that the profile is shallower than our glide ratio, we'll need some power to maintain it.

 

[clazman]

How much control do you have over your flight route, what influences your choices?

 

[AviatorInsight]

How much control do you have over your flight route, what influences your choices?

By flight route, do you mean what is on our flight plan? If so, then that is pre determined for us using software that identifies the quickest flying time and is also the most fuel economical at the same time. For example, the plan may have us flying south of the direct track for a couple of hundred miles to pick up a 150kt jet stream. Flight Planners will also build into the system, diversions around cyclones, and diversions from known volcanoes which have been erupting around the vicinity of the planned track.

Once in the air however, we then determine the aircraft's path with what we see on the radar. We will request from ATC a number of miles left or right (in the case across the Pacific, both) and once approved, this then lets us manoeuvre as required around the weather. Once clear we will usually either regain the pre-programmed track or just get direct tracking to the next waypoint along the plan and continue on.

Hope that answers your question.

 

[por930]

Hi AviatorInsight, you are new here, Welcome. Can you give us a rundown what you fly, domestic or international, common routes...etc

 

[AviatorInsight]

Hi AviatorInsight, you are new here, Welcome. Can you give us a rundown what you fly, domestic or international, common routes...etc

Thanks por930, glad to be here. I'm on the B777, predominately operating on the Pacific or Middle East runs. That should narrow it down somewhat.

I'll try contribute as much as I can. I'm very passionate about aviation and has been part of my life ever since I can remember. Questions don't have to be just technical, anything to do with the lifestyle, I will be happy to answer for you as best I can.

I look forward to contributing to this thread and others.

 

[Skyring]

Thanks por930, glad to be here. I'm on the B777, predominately operating on the Pacific or Middle East runs. That should narrow it down somewhat.

I'll try contribute as much as I can. I'm very passionate about aviation and has been part of my life ever since I can remember. Questions don't have to be just technical, anything to do with the lifestyle, I will be happy to answer for you as best I can.

I look forward to contributing to this thread and others.

Very glad to have you aboard, AviatorInsight! This thread has got to be the most fascinating and informative on the site, and I look forward to reading your contributions.

Speaking of lifestyle, how accurate is this response to a question on Quora?

Flippancy aside, the B777 seems to be a remarkably efficient and safe aircraft - most of the incidents and accidents involving this type have been due to external factors, and only one - the SFO crash - can be traced to the coughpit and its systems.

What is this plane like to fly? A handful, a doddle, or something in between?

 

[AviatorInsight]

Very glad to have you aboard, AviatorInsight! This thread has got to be the most fascinating and informative on the site, and I look forward to reading your contributions.

Speaking of lifestyle, how accurate is this response to a question on Quora?

Thanks for the welcome Skyring. Firstly, Wow! Talk about being thrown into the deep end! But yeah about 2 - 3 times TransPac sounds about right!

Flippancy aside, the B777 seems to be a remarkably efficient and safe aircraft - most of the incidents and accidents involving this type have been due to external factors, and only one - the SFO crash - can be traced to the coughpit and its systems.

What is this plane like to fly? A handful, a doddle, or something in between?

Very efficient and safe for sure. According to the preliminary report of the recent DXB crash, that can also be traced back to the aircraft systems. Mode awareness is a trait that is absolutely essential in today's modern flight decks.

As far as the handling goes, it is a pleasure to fly! Very responsive, and performs well at all stages of the flight (during a step climb, up to 2000ft/min fully loaded at altitude). The fly-by-wire certainly makes it easy, and a lot of the systems having an auto function, eliminates a lot of the switch flicking you'd otherwise see on an older aircraft (B737 for example).

 

[jb747]

How much control do you have over your flight route, what influences your choices?

We generally have nothing to do with it. It's built by a planning department that will weigh all sorts of costs, be they time, fuel, or overflight, in an effort to build the least cost flight for the day. Routes across the Pacific could be over a thousand miles apart on consecutive days.

Back when we were flying over Afghanistan, it was sometimes strategically advantageous to change from the route you were planned on, to one of the alternatives, and we'd just arrange that with the controllers at the time.

 

[Quickstatus]

Jb747 and others:
What's the average difference in flown distance on your usual routes?.

 

[docjames]

Back when we were flying over Afghanistan, it was sometimes strategically advantageous to change from the route you were planned on, to one of the alternatives, and we'd just arrange that with the controllers at the time.

Advantageous to avoid other civilian traffic (get into a better/earlier "slot"), or advantageous meaning "safer" from military traffic or other?

 

[AviatorInsight]

Jb747 and others:
What's the average difference in flown distance on your usual routes?.

Sorry Quickstatus, maybe it's the scotch talking, but I'm not sure I understand your question?

 

[Flashback]

Sorry Quickstatus, maybe it's the scotch talking, but I'm not sure I understand your question?

I think the question might be in actual distance flown vs what the "as the crow flies" distance is, maybe?

 

[AviatorInsight]

Thanks Flashback, that was my initial thought too. Well the great circle track (which is the shortest distance between two points on a sphere) on a SYD-LAX is around 6500nm.

To be honest I haven't really taken that much notice, but on the plan it's usually around 6650nm (including any departure and arrival tracking). But just out of interest, today's VA1 is tracking well south of the great circle out of Australia with a total distance of 6750nm.

 

[Milefest]

We generally have nothing to do with it. It's built by a planning department that will weigh all sorts of costs, be they time, fuel, or overflight, in an effort to build the least cost flight for the day. Routes across the Pacific could be over a thousand miles apart on consecutive days.

Silly question, what's overflight?

 

[jb747]

We don't compare the plan to the great circle track. The plan distance is used as a crosscheck against the FMC load. Any FMC error will also show up as an erroneous arrival fuel figure..which is also checked.

Looking at a pair of 93/94 plans (on the same day). The great circle distance between Melbourne and LA is 6883 NM. The 93 has a distance of 6954 and a time of 13:25, whilst the 94 has 6948 and 14:55. Arrival and departure tracking is not accounted for on the plan.

 

[jb747]

Silly question, what's overflight?

There are no silly questions, but probably some silly answers.

Every country charges you for flying through their airspace. Sometimes these charges are high enough that actively avoiding them, even it costs fuel, is worthwhile. For quite some time most western airlines were not flying through Iranian airspace...not because of any safety issues (they have quite good ATC), but because the sanctions meant there was no legitimate way of paying their fees.