737 'Tailstrike' Caused By Typo On a Tablet (arstechnica.com)

An anonymous reader writes: In August of last year, a Boeing 737 operated by Qantas experienced a tailstrike while taking off — the thrust wasn't great enough for the tail to clear the runway, so it clipped the ground. The investigation into the incident (PDF) has finally been completed, and it found the cause of the accident: the co-pilot accidentally entered the wrong plane weight data into the iPad used to make calculations about the takeoff thrust. "First, when working out the plane's takeoff weight on a notepad, the captain forgot to carry the "1," resulting in an erroneous weight of 66,400kg rather than 76,400kg. Second, the co-pilot made a "transposition error" when carrying out the same calculation on the Qantas on-board performance tool (OPT)—an iPad app for calculating takeoff speed, amongst other things. "Transposition error" is an investigatory euphemism for "he accidentally hit 6 on the keyboard rather than 7." This caused the problem: "For a weight of 76,400kg and temperature of 35C, the engine thrust should've been set at 93.1 percent with a takeoff speed of 157 knots; instead, due to the errors, the thrust was set to 88.4 percent and takeoff speed was 146 knots."

Comments

Data data everywhere and not a drop to think

[burtosis]

It still boggles my mind how we live in the Information Age and this data was not automatically uploaded and calculated. I'm not saying it dosent require a human to sign off on, but it's mildly insane it isn't all automatically calculated and simply checked.

Re:Data data everywhere and not a drop to think

[Anonymous Coward]

I don't know for sure, but I believe the pilots are supposed to *only* use the certified avionics systems in the aircraft, but they hate the godawful UIs in those things so they use iPads even though they're really, really, really not supposed to.

Re:Data data everywhere and not a drop to think

[iONiUM]

Uh, isn't the weight the plane + fuel + baggage + passengers? 3 of those are clearly easily gathered (fuel, plane, baggage) automatically. Only the 'passengers' part isn't, but I'm assuming they just use averages (they don't weight us when we get on) via # of people * average weight.

So yea, why the fuck isn't this automatically calculated and prepared? What part of this requires a human who is bad at math and able to make mistakes on simple data input?

Re:Data data everywhere and not a drop to think

[es330td]

Is there some reason they shouldn't just use 100% thrust at takeoff and make sure the cargo being carried was less than the maximum capacity?

These engines are optimized for certain turbine speeds. By staying within the recommended ranges they reduce wear & tear and improve efficiency. In addition, max acceleration is harder on the passengers. Think of it like driving a car. Do you accelerate from every stop by pressing the gas pedal all the way to the floor or do you match how much gas you give it to the driving conditions and who and how many people are in your car?

Re:Data data everywhere and not a drop to think

[gstoddart]

Is there some reason you don't redline your car before you pull away from a stop light?

Having pilots who don't know how to do anything other that "rev it up to full speed and let 'er rip" sounds like a terrible idea to me.

Both for maximizing fuel and not abusing the engines, doing it based on real numbers makes more sense than the equivalent of flooring it.

Re:Data data everywhere and not a drop to think

[Anonymous Coward]

And $745,000 of that is needed to comply with and get the app through DO-178C certification. The rest is pure profit.

Re:Data data everywhere and not a drop to think

[SuricouRaven]

You shouldn't even need to calculate it - the plane can measure weight itsself. There are only three points of contact with the ground, easy to mount a strain gauge in the landing gear. It might not be precise enough, but it'll be fine for sanity checking - if the measured and calculate weights differ by more than the margin of error, plane refuses to bring the engines above taxi thrust.

Re:Data data everywhere and not a drop to think

[Goldenhawk]

I'm a flight test engineer that works on large (passenger-class) aircraft. We do tests related to this issue.

It actually has a lot to do with takeoff safety, ironically enough. If you lose an engine at high power, the airplane will try to yaw (turn left or right) because the engine(s) on the other side are still producing thrust. At lower speeds, with less aerodynamic forces, the rudder is not capable of keeping the airplane in a straight line. So there's a speed called "Vmcg" - "velocity minimum control ground" - below which you MUST pull back the power on the good engines to avoid going off the side of the runway (you're going to have to stop the takeoff). There's also a speed called Vmca, the airborne minimum control speed (you will start to yaw out of control).

So with less power on all the engines, there is less asymmetry possible in the event of a failure. With reduced takeoff thrust, you don't need as much rudder at any given airspeed, so your Vmcg and Vmca are both lower.

This is important for takeoff because if you have a lot of runway available, you can use it by taking longer to accelerate (by having lower thrust). As a consequence, your risk in the event of an engine failure is reduced - you won't head off into the grass if it happens on the ground, and you'll be assured of sufficient control authority if it happens in the air.

So when an airplane manufacturer builds the "takeoff performance charts", these Vmc speeds heavily factor into the takeoff planning.

Now, in this tail strike mishap, the lower weight caused the iPad to compute TOO LOW a speed. Lifting off too slow takes more nose-up (pitch) angle; lift goes up as a linear function of pitch angle; lift has to equal weight to go flying. Because of the reduced takeoff thrust, they were already planning to use most of the runway to accelerate - which put them into a corner; they were too slow to take off at the normal pitch angle, but were out of room to stop. So they pulled up until the airplane started flying - which means they pulled up high enough that the tail hit the ground (just barely in this case).

Re:Data data everywhere and not a drop to think

[jbwolfe]

As long as the device and software (the iPad and app in this case) has been vetted by the regulating agency, it is acceptable to use and may actually be required. I'm unsure of the interface to which you are referring, but takeoff performance calculations are not integrated into any large transport aircraft that I have ever flown. The FMC can only calculate data accurately if its given the correct inputs- stuff like ZFW, CG, flap setting, reduced thrust setting, etc. GIGO

At my airline, takeoff data calculations are centralized (acquired through datalink) rather than carried onboard, but still require those variables.

Re:Data data everywhere and not a drop to think

[ShanghaiBill]

Ok, Einstein. Provide model numbers of the sensors capable of doing what you claim "are clearly easily gathered".

The landing gear use hydraulic and pneumatic systems, and there are already sensors that measure that pressure. The weight of the plane is a simple linear function of the pressure of the fluids in the landing gear.

Re:Data data everywhere and not a drop to think

[Obfuscant]

I wish I had mod points today. Excellent answer.

Add in the safety factor that a heavy aircraft is better lifting off further down the runway so that the following departures and arrivals of lighter aircraft are less likely to encounter wake turbulence. As a small plane jockey, you always want to take off or land before the point the previous heavy aircraft took off, or after the point the previous heavy aircraft landed.

Re:Data data everywhere and not a drop to think

[MouseR]

The iPad is not at fault here. Pilot did simple math and forgot to carry over a "1". There's no carry over when you let a software add.

It's a whole system failure: paper being handed out to be hand-computer and then the number punched into a iPad for final trust numbers which are then entered in the avionics system.

A frickin piezo on the landing gears would have done the trick.

Re:Data data everywhere and not a drop to think

[Andy Dodd]

The real story behind $10,000 hammers you hear people joke about in defense & aerospace:

It's a $500 hammer because it's made from premium materials and is designed to handle things most hammers in civilian use would never see, plus $9,500 worth of government-mandated paperwork proving you didn't rip off the government.

Re:Data data everywhere and not a drop to think

[leathered]

I'm only a private pilot but you also need to take into account temperature, air pressure (QNH), headwind component, the amount of runway you have to work with as well as weight to calculate takeoff performance.

Re:Data data everywhere and not a drop to think

[DarkOx]

I am not a pilot so I don't know but I am curious. In all the old war novels I read they just run the throttles to "take off power" and go.

Is there any reason you don't just slam the throttles open on a modern jetliner until you are off the ground? Why do anything other than make sure you are not 'to heavy' to make it before you run out of runway? What not just 'punch it'.

Re:Data data everywhere and not a drop to think

[Lisandro]

There's a good number of reasons for this, and they're not immediately obvious.

- Optimize fuel usage; you want to burn only as much as you need on takeoff
- Reduce engine wear as much as possible. Engine overhauls are awfully expensive for airlines.
- Don't overthrust engines in case of failure. If an engine goes out on takeoff with too much airspeed the airplane might not be able to correct yaw before running out of runway.

On small piston aircrafts, yes, you usually go full power on takeoff every time. Airlines are a very different beast though.

Why do these data need to be entered manually?

[QuietLagoon]

Where do the pilots get the data? Is it displayed to them? Is it shouted at them as they board the plane?

.
Why can't the source of the data convey the data to the tablet apps automatically? Why involve an error-prone human in the process?

Re:Why do these data need to be entered manually?

[Solandri]

Because if an automatic system fails in an "obvious" way and causes an accident, people here would be posting "why wasn't a human in the loop?"

In an ideal world, both automatic systems and people would complement each other with a person manually checking the automatic results. But in the real world, once you automate the system, people tend to get lazy and stop double-checking the automated system's calculations. NASA ran across the same problem - in the post-Challenger investigation they discovered that having multiple inspections actually decreased safety. Each inspector assumed the other was doing his job, so became more lax and sometimes didn't check things thoroughly or sometimes even skipped checks. Even in this incident, the pilot using the iPad didn't bother checking to make sure the number he entered into the iPad was the number he thought he typed. He just assumed the input method worked, despite everyone who's tried to type an email on a touch interface knowing that errors occur frequently.

So you end up with an either/or situation. Either the system has to be completely automated with the engineers trying to think up every possible scenario during design, and it'll still occasionally fail in ways you never thought of which will lead people to question why humans were left out of the loop. Or it has to be reliant on humans doing everything by hand, to ensure they take their job seriously and actually do it. Unless an automatic system has a track record showing decades of reliability (this is why computers on aircraft are usually more than a decade old), aerospace usually relies on humans.

Re:Why do these data need to be entered manually?

[jbwolfe]

As a unionized airline pilot, I can assure you that your statement is completely false. I'd suggest that you visit here: http://alpa.org/ to get a better idea about the kinds of things that union has done to improve safety in the industry. Such as the following: science based duty limits, TCAS, Captains authority, security of the cockpit, hazardous cargo, safety reporting system, etc.

I suspect, sadly, that your bias against unions is an indicator that your mind is made up already.

So wrong data in Flight Management Controller

[Anonymous Coward]

So somebody entered bad data into the Flight Management Controller.

Happens all the time with Qantas, which has been caught out for having extremely lax protocols.

Search the ATSB database, there have been more than a dozen incidents in the past decade which by pure luck didn't result in mass casualties.

Then read the BS excuses Qantas gives. Such as "the ladder wasn't tall enough to check the engine cowling was locked".

As if...

[CanadianMacFan]

As if the co-pilot couldn't have made the exact same mistake with a calculator or even paper.

Why is a tablet signiifcant to this story?

[JoeyRox]

They could have just as easily mistyped the weight using the on-board flight management system's keyboard.

Where is there check?

[Rinikusu]

I used to be a Ramp Agent at a major international shipping firm. We did weight and balance for the flights. We had several layers of redundancy for our numbers: Every container number and weight was rechecked by another Ramp Agent, and then once again at the gate to match with the load sheets. We realized if we put the numbers in wrong, that could result in loss of life (not to mention aircraft assets and cargo). We took this job very seriously. Once we turned that paperwork (now done via ACARS, supposedly), I would hate to think that the cockpit just fat fingered the numbers in on their end without having a secondary check. "Hey Captain, can we check the numbers real quick?" Probably take them 15-30 seconds at most since they'd be concerned with big picture numbers and not individual positions.

Re:It's even worse than that now.

[Austerity Empowers]

That'll make for some angry birds.

Re:That's why...

[BZ]

They did both do the calculation. The pilot did the arithmetic wrong and the copilot typed in his result wrong, and the upshot was that the numbers they entered independently agreed with each other... and were both wrong.

Qantas

[Anonymous Coward]

On a recent flight from SYD to MEL our Qantas pilot ranted on the PA about how evil the ATSB was because they wouldn't let the plane take off, because Qantas engineering accidentally put an incorrect black box into our plane. He said that Qantas maintenance made mistakes like that all the time, and that, since it wasn't a safety issue to fly without a black box, the ATSB were just being pricks.

I will never set foot on a Qantas plane ever agin.

Re:5%

[Talderas]

Headwinds are helpful for takeoff. Tailwinds are detrimental.

Re:It's even worse than that now.

[sycodon]

Umm...Tail Strikes on take off are caused by over rotation.

Regardless, seems to me that pilots calculating the take off weight is anachronistic. Manufacturers can easily incorporate weight sensing devices into the gear. Or, weight sensors could be incorporated into a Wheel Chock device. There is no reason for guesstimates.

Re:It's even worse than that now.

[tripleevenfall]

"76,860 kg" autocorrected to "Settings thousand ate sexy cardiograms"

Re:amazing no ground scale or even strain gauges

[Idarubicin]

...but then the stupidity of taking off at less than 100% throttle to save a little bit of fuel at the expense of increasing risk is also a pretty dumb thing to do, engineering wise.

Taking off at less than 100% throttle means reduced acceleration, which reduces stress on the airframe (and passengers). It reduces wear on the engines and - more important - reduces the risk of turbine failure. It makes the aircraft easier to control (less unbalanced thrust) if it does lose an engine immediately before or after takeoff.

So...not just to save fuel.

Re:It's even worse than that now.

[michelcolman]

First of all, tail strikes on take-off are obviously the result of overrotation, but this usually happens because the pilot rotates at the wrong speed. You pull back, expecting the plane to leave the ground, but instead the plane remains on the ground while the nose keeps going up. Also, you may be running out of runway if the calculations were off, so you'll pull back regardless.

About the weight sensors: good idea, but this is aviation, where everything has to work reliably in pretty difficult environmental circumstances. Even something as simple as a proximity switch to determine whether or not the gear is down, fails from time to time. We often deal with incorrect tire pressure indications, temperature indications, etcetera. Measuring the weight of a plane with sufficient precision is quite a bit more complex than a simple tire pressure reading, so I can't see any manufacturer trusting that kind of system enough to let it determine take-off settings by itself. Maybe as an extra crosscheck for the data from the loadsheet, sure, but not as the primary source of information.

People always go "we should replace the pilots with automated systems because pilots make too many mistakes", but they have no idea how many mechanical failures we deal with as part of the routine of our job. We make mistakes, sure. But so does automation.