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最近比较忙，还没有时间翻译，大家可以用谷歌翻译，图文并茂先了解一下。 In this second post about the 757 we're looking into the redundancy of various systems needed to navigate the plane.
I left you in the last post with a view of the IRS control panel in the overhead:
The three IRS units feed three independent computers called "symbol generators" that create the images for the EFIS screens. The left symbol generator feeds the captain side EFIS, the right symbol generator the f/o side EFIS, and the center symbol generator can be selected from either side as backup. The left IRS feeds the left and center SG, the right IRS the right SG. The center IRS can be switched to either side SG as backup.
The symbol generators are individually programmed, which means you can independently control the navigational map displays of both captain's side and f/o's side.
Remember the instrument comparator system I talked about in the last post?
It not only compares the altimeters, it also compares pitch and bank information from the IRS units.
That gives you a very useful hint in the case of an otherwise hard-to-detect type of IRS malfunction: An excessive drift, that slowly accelerates. The excessive drift is much more serious than a complete failure of an IRS unit (which you can simply detect by all information on the respective side going away and the FAULT light illuminating): you will normally not notice this until the FMC detects a gross mismatch of IRS positions. There's however a chance that not only the position drifts away but also the attitude information. In that case, the fault will be detected by the instrument comparator and announced on the PFD:
The comparator tells you there is something wrong. By comparing the outputs of the three IRS units in the overhead we can quickly tell that it is the left unit that is wrong, because C and R agree on our speed being 0 as we are on the ground while I do this exercise:
 
So we decide to turn off the left IRS which leaves us with the captains' side EFIS rendered pretty useless and a NO LAND3 indication that I will explain later (Of course the VSI should also display an INOP flag. I corrected this shortly after I made those screenshots):
Luckily, there exists the instrument source switching panel. So why not feed the left symbol generator from the center IRS?
That brings us back the indications on our EFIS. Also, it will have brought back another indicator that you probably didn't even notice was out of order: The f/o side RDMI. The reason is the RDMIs are cross-wired between captain's and f/o's side, so a left IRS off will cause the right RDMI to fail before you select the ALTN IRS source on the captain's side:
With the captains IRS source selector on alternate, both captain's SG and F/O's RDMI are now working. But the autoland annunciator remains in "NO LAND3". What does this instrument tell us anyway?
The autoland annunciator tells us the autoland capability of our plane.
With three IRS units working, two generators working, and all hydraulic systems working, we are triple-channel autoland capable, we are in a state that is called "fail operational". That is, an autoland is possible even in case of a failure of one channel, like an autopilot or IRS unit quitting just before touch-down.
Now with one IRS inoperative, our autoland capability goes down from "fail operational" to "fail passive". We can only conduct a two-channel autoland now, and we'll completely lose autoland capability in case another channel fails.
Besides a single IRS failure, what else can cause "fail passive" autoland capability? For example, a single generator failure. We are then also fail passive because we have only two sources of electricity (one generator and the standby DC inverter).
In that case, starting the APU would give us three independent sources again and we could upgrade to fail operational (given the three IRS and APs were working).
In case of more failures, like dual IRS failure, dual generator failure or dual hydraulic failure we lose autoland capability altogether. In that case, the autoland annunciator will very clearly tell you "NO AUTOLND"!
So whenever you realize an autopilot refuses to arm for autoland, check the autoland capability annunciator.
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发表于 2013-6-10 12:34:02
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发表于 2013-6-12 12:46:03
