Saturday, September 12, 2009

How long until Australia suffers a large-scale air crash?

Judging from this episode, not long.

For a few terrifying moments, those on duty in the airport's control tower had no way of knowing the fate of Emirates Airlines flight 407 after it limped into the air and flew low over houses in the densely populated suburb of Keilor on March 20.

The Emirates plane, bound for Dubai, struggled to take off and then gain altitude after one of its pilots wrongly calculated the weight of the aircraft by 100tonnes.

The tail of the plane hit the runway five times before the captain ordered full thrust at the last minute to lift the Airbus over the airport perimeter fence, knocking out a strobe light and an antenna on the way.


Zardoz said...

According to the spec sheet for the A340-500 the empty weight of the aircraft is 170 metric tons and it has a max take-off weight of 380 metric tons. Just how does one make a calculation error of 100 metric tons and not notice it? Put the decimal point in the wrong place, did we?

Oh well, at least they pilots will be able to hire on to one of the regional airlines here in the US without any problem.

Skeeter said...

That was certainly the closest we have had to a truly major disaster at an Australian airport. The aircraft left wheel tracks in the grass at the end of the runway, and clipped a small antenna building 300 metres after leaving the runway. So the pilots managed to avoid a large ball of flame by the skin of their tail.
Keilor was not threatened by a "limping" aircraft. The first buildings beneath the flight path are 4 kms south of the runway end and the aircraft performed normally once they got airborne.

It was not a decimal point in the wrong place but a wrong keystroke (1 instead of 2) when entering the 100s of tonnes fuel-load into the computer.
But Zardoz is right in saying that the error should have been noticed by the two pilots.
That's why that take-off was the last one they made as airline pilots.

A horrendous near-miss like this does not make it more likely that an air disaster in Australia is imminent. In fact, the reverse is true because lessons are learnt and changes are made.

kae said...

Hi Zardoz, thanks for your input, there wasn't a lot of information in the news report.

Hi Skeeter, my concern is why the incident happened, and is it a sign of lack of training of pilots and other people responsible down the line for checking the information? It is frightening that the plane dragged it's tail along the runway before lifting off. It's a miracle that damage wasn't done which would have compromised the aircraft in flight - surely whacking the tail on the ground a few times on take off isn't good for the aircraft!

Thanks for easing my concerns by explaining that this will, if anything, ensure that an incident like this won't happen again as checks and balances will be put in procedures.

Skeeter said...

Hi Kae. Lack of training is unlikely to be the cause, and the airline would have checking procedures in place. There is little doubt that those checks were not made properly by the pilots concerned.
Such a low weight for such a long flight should have rung alarm bells in the heads of everyone on the flight deck.

How did it happen?
The take-off performance of an aircraft depends on how heavy it is at the start of the take-off roll. The heavier the aircraft is, the faster it must be moving to generate enough lift to get airborne.
More runway length is needed to achieve higher take-off speeds; and the aircraft engines need to produce more thrust to accelerate a heavier aircraft to the higher speeds.
Other factors that affect the runway-length needed for take-off are air temperature and the wind component on the nose. (A headwind while you are on the ground = free airspeed.)
All of these factors and others are entered into calculations for every airline take-off.
The take-off weight (TOW) for a modern airliner can vary by large amounts. As Zardos has pointed out, in the A340-500, TOW can be anywhere between 170 and 380 tonnes, depending on the amount of fuel and payload.
At lower weights, the engines' maximum thrust is not required for take-off. The take-off calculations include the amount of thrust required, and the engine throttles are set to achieve that lesser amount for lower TOWs.
What went wrong at Tulla was that the throttles were set for a weight that was 100 tonnes less than the actual weight.
Various critical speeds are also calculated for the take-off. For example; the "rotate" speed (Vr). This is the speed at which the pilot raises the nosewheel off the ground and "rotates" the aircraft into the nose-up attitude needed to generate lift.
If the Vr is correctly calculated for the weight, it will be achieved in good time before the end of the runway, and the aircraft will fly off the ground during the rotate.
In the Tulla incident, the Vr should have been much faster than the one calculated and more engine-thrust should have been used to achieve the higher Vr before the end of the runway.
When the pilot attempted to rotate, his low speed meant there was not enough lift to get the actual weight airborne, and the tail struck the ground.

The most likely outcome at this point is flaming wreckage spread over several hundred metres off the end of the runway.
They were extremely fortunate that the aircraft did not sustain major damage during the 300 metres it took them to get airborne and clear obstacles after they left the sealed runway.

kae said...

Hi Skeeter
Didn't the report say that the tail hit the ground several times?

Boy on a bike said...

For lots of useful explanations on all things air-crafty, read this guy:

Skeeter said...

Sorry for my absence, Kae; I have been involved in rural matters.
Yeah, the multiple tail strikes have been mentioned in reports. Not sure about it but probably a bit of bounce and panicky over-control involved as the task of getting airborne became ever more daunting.

BoaB, I often enjoy Captain Dave's site.
I retired in 1986 and there have been a couple of generational changes in technology since then, but what he writes is very evocative for me.
Makes me wish I had written about my experiences the way he does, instead of wasting all those hours in bars around the world, swapping stories with fellow pilots.

kae said...

Hi Skeeter
Glad you're still around!
It's not too late to write that book!