Boeing returns to drawing board on portion of 787 wing fix

reply to post by RichardPrice


Thank you for your professional explanation.

What's really not clear to me is whether the pilots action or the autopilot system action resulted in the breakup of the aircraft. I guess it's all just speculation as the real data may never be found.

I realize the pitot tube data is considered the best source of real information, but what I was getting at is perhaps the software could of been designed to compare it with other perhaps less reliable data such as GPS & other instrumentation. Certainly you can't account for every possibility, but if something like this happens I would think that they would at least look at it.

Again thanks for taking the time to explain and doing it in a manner that makes for a friendlier forum here.

reply to post by Harlequin


You make a valid point - perhaps the whole industry needs look at this problem closer. Thank you.

Here's a bit of an off topic post but since we seem to have some AC engineers / designers in the thread -

I'm doing research for a novel I'm working on and could use 5 minutes time w/ someone who has some expertise w/ rotary aircraft and thrust vectoring systems. My storyline involves a new aircraft design and I want to see how realistic my idea is.

And more on topic (at least w/ parts of the discussion)
We've discussed a few crashes which involved failures of the navigation systems, I'm still curious if GPS based systems can provide a backup. Is there a limitation on altitude readings w/ GPS?

Originally posted by ecoparity
I'm still curious if GPS based systems can provide a backup. Is there a limitation on altitude readings w/ GPS?

This is all IIRC.

Airliners can navigate paths by GPS, but I have not seen any stock units capable of reading off altitude. Therefore, the airliners would probably have to supply more advanced units if they want such a function. The rest of this will assume that an airliner has a unit capable of returning an approximate altitude.

GPS systems are, first and foremost, limited by nature in their accuracy. The government imposes a certain threshold of accuracy which is forbidden for civilian uses and reserved for military applications (these differ between satellite systems). In addition, atmospheric effects can cause problems with precise signal measurements.

More importantly, I have seen no GPS receivers capable of reading off figures in real time. there's a few seconds delay between each refresh of the onscreen information, and in emergency situations that's far too long to make use of.

Originally posted by ecoparity
Here's a bit of an off topic post but since we seem to have some AC engineers / designers in the thread -

I'm doing research for a novel I'm working on and could use 5 minutes time w/ someone who has some expertise w/ rotary aircraft and thrust vectoring systems. My storyline involves a new aircraft design and I want to see how realistic my idea is.

If you PM me, I will help as much as I can.

And more on topic (at least w/ parts of the discussion)
We've discussed a few crashes which involved failures of the navigation systems, I'm still curious if GPS based systems can provide a backup. Is there a limitation on altitude readings w/ GPS?

There is a huge limitation on GPS usage, as I will explain in my next post.

Originally posted by Darkpr0

Originally posted by ecoparity
I'm still curious if GPS based systems can provide a backup. Is there a limitation on altitude readings w/ GPS?

This is all IIRC.

Airliners can navigate paths by GPS, but I have not seen any stock units capable of reading off altitude. Therefore, the airliners would probably have to supply more advanced units if they want such a function. The rest of this will assume that an airliner has a unit capable of returning an approximate altitude.

Most units should be able to supply altitude, its an integral part of the GPS system.

GPS works via triangulation through usage of known signals, and as a result you will always get a position within a sphere - most incar units assume a 0 altitude, as it has no purpose for their usage, but I have yet to see a handheld unit that does not display the calculated altitude (even the iPhone does).

GPS systems are, first and foremost, limited by nature in their accuracy. The government imposes a certain threshold of accuracy which is forbidden for civilian uses and reserved for military applications (these differ between satellite systems). In addition, atmospheric effects can cause problems with precise signal measurements.

The 'Selective Availability' system was turned off several years ago, and new GPS satellites do not have the ability to reenable SA - its off permanently.

SA was removed because it was pointless - civilian GPS unit manufacturers were including the ability to use Differential GPS signals, which removed the artificial error that SA introduced into the GPS system and made civilian units accurate down to the centimetre level - making SA useless.

Differential GPS sfixed ground stations, available all over the globe, as additional triangulation sources and data points - the signal it sent out was not under the US Department of Defence control, and as such had no SA error - thus any DGPS signal could be used to correct the SA error and remove the inaccuracy.

Secondly, atmosphere distortions have no effect on the GPS system, it is designed to handle it - about 2 years ago I made a post here that covered GPS in detail, how it works and why it works. Its complicated, but it makes sense. I suggest you look it up if you wish.

More importantly, I have seen no GPS receivers capable of reading off figures in real time. there's a few seconds delay between each refresh of the onscreen information, and in emergency situations that's far too long to make use of.

When used as a navigation aid, GPS never works in real time as its always used for differences between two data points.

The real reason why GPS is not used for detailed cross checking in aircraft navigation systems is that it simply does not give the right information.

With a GPS signal you can work out ground speed, but that is useless to an aircraft with a head or tail wind - an aircraft relies on indicated air speed for flight purposes, and knowing how fast it is travelling in relation to the ground has no bearing on flight systems.

There are also similar differences between altitude readings and rate of climb/descent readings - GPS does not give what an airliner needs to carry out a useful cross check.

Lastly, usage of GPS relies on an external source of data - you cannot guarantee that on an aircraft.

GPS is fantastic for navigation, but its lousy for flight systems.


[edit on 25/10/2009 by RichardPrice]

Wow, four whole pages. If I'd read it first I wouldn't have posted.

Superfluous coment removed

The delays to the 787 have certainly helped Airbus from a marketing perspective as the four year lead held by Boeing (thanks in part, let us not forget, to Airbus' own screwing up of the original A350 concept) given them what looked like an iron grip on this market.

Half that lead has now been cut away and some airlines are even making noises about cancelling their 787's and ordering A350's instead (no, I don't think so either, but they want Boeing to know how mad they are). What this means is that we might actually get some meaningful competition in the sector as the 737 and A320 have in the SA sector and this can only be good.

Any comments about Airbus being a new company or being less experienced than Boeing show an ignorance of the fact that Airbus is an amalgam of companies and that in that heritage are many jetliners from the 50's and 60's, the Airbus design team didn't just pop into existance, there is every bit as much design and engineering experience within Airbus as there is at Boeing, it was Airbus for instance who made the very first widebody twin, and Boeing who followed, just as an example.

Airbus claims they have learned from the A380 and A400 and have foreseen and prepared for the things that are now slowing Boeing down. We will just have to see if they are just fancy words or not, but I hope not.







[edit on 25-10-2009 by waynos]

I did some light searching and found the FAA's planning team for the Aircraft GPS standard and system, as I suspected its being worked into the next gen ATC initiative (which may enable the flying car thing eventually).

FAA - GPS

The main purpose they cite is the ability to do direct point to point navigation in place of waypoints. My thought that it would be a good back up in case of a failure in the onboard navigation system turns out to be already in practice, they have portable units which are specially made for pilots which are quite popular. I realized tonight just how new the user friendly and full featured models are, I was falling into that trap of thinking they had been around for a lot longer.