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Why Does 777 Have Glass "backup" ADI  
User currently offlineJhooper From United States of America, joined Dec 2001, 6206 posts, RR: 11
Posted (12 years 1 week 2 hours ago) and read 7270 times:

I was wondering why the 777's backup ADI is glass rather than a physical instrument such as in the 767. Isn't the idea of a backup instrument to be as redundant is possible? By that logic, you shouldn't have both depend on electricity. I know people say that it's impossible to have a total electrical failure on the 777, but I'd never say "impossible".

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User currently offlineN243NW From United States of America, joined Jul 2003, 1656 posts, RR: 19
Reply 1, posted (12 years 1 week 2 hours ago) and read 7261 times:

I too realize that a total electrical system failure in a 777 is possible, but it is extremely unlikely. Plus, the electronic backups are probably more reliable than their mechanical counterparts anyway.
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User currently offlineCPH-R From Denmark, joined May 2001, 6305 posts, RR: 3
Reply 2, posted (12 years 1 week 1 hour ago) and read 7243 times:

But still, an analogue instrument would still have a very hard time to break down, compared to a EADI

[Edited 2003-08-26 20:55:27]

User currently offlineCancidas From Poland, joined Jul 2003, 4112 posts, RR: 10
Reply 3, posted (12 years 1 week ago) and read 7230 times:

they are requred by the FAA, i'm pretty sure.

i once got caught using a garmin gpsmap 196 inflight. when i talked with the pilots later they explained to me not only why it was taken away from me but also that electronic instrumentation does update quicker that the instruments on board the a/c. shocked, i tried it in my 172. sure enough, it did update faster and was more acurate than the anologs.

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User currently offlineVC-10 From United Kingdom, joined Oct 1999, 3719 posts, RR: 32
Reply 4, posted (12 years 6 days 23 hours ago) and read 7222 times:
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I too realize that a total electrical system failure in a 777 is possible, but it is extremely unlikely. Plus, the electronic backups are probably more reliable than their mechanical counterparts anyway.

Correct, they are more reliable than a mechanical stby instrument. Don't forget a mechanical stby ADI also has a gyro in it so it too needs electrical power. LCD's consume less power than conventional instrument

User currently offlineDash8tech From United States of America, joined Jul 1999, 732 posts, RR: 5
Reply 5, posted (12 years 6 days 21 hours ago) and read 7191 times:

It probably (dash8tech, not 777tech) runs off of the battery bus and thus with a complete electrical failiure it's still running off the batts.

User currently offlineLstc From Canada, joined Jun 2003, 320 posts, RR: 0
Reply 6, posted (12 years 6 days 18 hours ago) and read 7145 times:

But still, an analogue instrument would still have a very hard time to break down, compared to a EADI

This was the case not too long ago. Its taken a long time, but these electronic instruments have finally achieved a reliability level that makes them suitable as a standby instrument but with MTBF rates that are still slightly lower than many mechanical instruments.

FAR 25 requires that the loss of all sources of altitude or heading or attitude or airspeed must be extremely improbable. That includes the standby and main instruments. The term "extremely improbable" relates to a failure rate of 1x10E-09 per flight hour. So in many cases, to bring this failure rate to an acceptable level while using a less reliable electronic instrument, the main instruments may have to be a more robust (expensive) type or they may be a requirement to raise the redundancy. ie: 3 main altimeters instead of 2.

Of course the electronic standby instrument must be paired with a power supply that is independant of the aircraft generating system. This is usually in the form of a battery pack that is charged, but isolated from the main aircraft electrical system.

The battery packs must be capable of supplying power to the standby instruments for times varying from 30 minutes to a few hours depending on the operational approval sought.

It is interesting to note that the standby instruments must become operational with no flight crew action following a failure of the electrical system.

User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 25
Reply 7, posted (12 years 6 days 16 hours ago) and read 7120 times:

The B777, is of course different than most other airplanes. There is a SAARU (Secondary Attitude Air Data reference Unit) that contains the gyros and processes all of the information. It then sends this info to the Standby Attitude Indicator. The indicator is just a display...there is no gyro inside. The SAARU is powered by 28VDC. So in theory (as it has been said already), it is the same as other aircraft. You need 28VDC regardless.

User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 8, posted (12 years 6 days 12 hours ago) and read 7092 times:

There are even certified aircraft with no backup instrumentation visible. The backups are built into the displays, driven directly and will pop up automagically if the normal instrumentation fails, of at the press of a button.

How you do it is secondary to showing that it can provide the demanded reliability and redundancy!


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User currently offlineMD-11 forever From , joined Dec 1969, posts, RR:
Reply 9, posted (12 years 6 days 12 hours ago) and read 7092 times:

The MD-11's operated by Swissair and later on Swiss also are equipped with a "glass backup instrument". The reason for this upgrade was the lesson learned from the SR111 accident. One finding was, that the position as well as the visibility of the standard backup instrumentation is not satisfying in case of smoke in the cockpit.

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User currently offlineUTA_flyinghigh From Tunisia, joined Oct 2001, 6495 posts, RR: 48
Reply 10, posted (12 years 6 days 11 hours ago) and read 7081 times:

As you can see, even some classic 737's have glass backup instruments :
I don't know if that particular aircraft is an exception though.

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User currently offlineAJ From Australia, joined Nov 1999, 2403 posts, RR: 24
Reply 11, posted (12 years 6 days 11 hours ago) and read 7082 times:

The Boeing 747-400ER has also incorporated a LCD standby ADI (as well as the other electronic displays).

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User currently offlineLstc From Canada, joined Jun 2003, 320 posts, RR: 0
Reply 12, posted (12 years 6 days 6 hours ago) and read 7032 times:


I'm curious. Is the "certified" aircraft you are talking about a FAR/JAR 25 aircraft or a GA aircraft?

I have a hard time believing you can meet the FAR/JAR 25 regulations with such a setup:

Sec. 25.1333

[Instrument systems.]

[For systems that operate the instruments required by Sec. 25.1303(b) which are located at each pilot's station--
(a) Means must be provided to connect the required instruments at the first pilot's station to operating systems which are independent of the operating systems at other flight crew stations, or other equipment.
(b) The equipment, systems, and installations must be designed so that one display of the information essential to the safety of flight which is provided by the instruments, including attitude, direction, airspeed, and altitude will remain available to the pilots, without additional crewmember action, after any single failure or combination of failures that is not shown to be extremely improbable; and
(c) Additional instruments, systems, or equipment may not be connected to the operating systems for the required instruments, unless provisions are made to ensure the continued normal functioning of the required instruments in the event of any malfunction of the additional instruments, systems, or equipment which is not shown to be extremely improbable.]

User currently offlineCx flyboy From Hong Kong, joined Dec 1999, 6726 posts, RR: 55
Reply 13, posted (12 years 4 days 6 hours ago) and read 6911 times:

In our 777s we have two standby instruments, one AH, and the ASI. The 777-300ER aparently has one single intergrated standby instrument just like a mini PFD, and we are considering having this retrofitted to our current 777s.

User currently offlineS.p.a.s. From Liechtenstein, joined Mar 2001, 980 posts, RR: 2
Reply 14, posted (12 years 4 days 5 hours ago) and read 6900 times:

Speaking about the backup ADI from the MD11, I made this same question to a friend who happens to be a F/O with Swiss and he told me that indeed it provides better reading and also no parallax errors.. About power source, he told me that it is connected to the INS 2 battery and to the plane's battery bus.

Lufti(LH) is also installing those to their MD11's and perhaps it will be standard retrofit from now on for another MD11s around.



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User currently offlineCrosswind From United Kingdom, joined Nov 2000, 2622 posts, RR: 57
Reply 15, posted (12 years 3 days 22 hours ago) and read 6877 times:
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Newer B757s (so I would assume this also applies to 767s) have an electronic standby ADI. It appears to be an option on any new-build Boeing aircraft...


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