747-MD11 design

[Guest Moonraker]

The MD11 is my favourite FS-airplane at the moment because it is so different to the Boeing aircrafts (as is the MD80). While I enjoy to learn about those differences, I still have to say I prefer the layout/logic of the Boeing-cockpits and their versions of FMCs. Maybe it is simply by custom, because I have grown accustomed to the Boeing philosophy - I don

[downscc 1,327]

The number of backup systems alone doesn't necessarily improve reliability. A twin engine has twice the probability of an engine failure and engine out failures in twins are more likely to be fatal than single engine failures. In this case, the real improvement in reliability comes from pilot proficiency with single engine operations.Three or four hydraulics statement without understanding how the systems are interrelated is an oversimplification. Two controllers can be safer than three depending on how the engineers design for covert failures (an undetected failure). If the probability of covert failures is very small then two controllers will have a higher availability than three controllers, especially if the three are configured in a two-out-of-three voting scheme. This scheme requires that all three be operating to achieve required reliability whereas the one-out-of-two scheme will be almost as "safe" with one failure (give covert failures are nil). This is a very complex and interesting engineering problem, one that I have dealt with in my job as an engineer and there is an engineering field called reliability engineering that works with this body of knowledge full time. The early DC10s had a problem with hydraulics but the MD11 has addressed those failure modes and is considered state-of-the-art "safe."

[Guest Moonraker]

The number of backup systems alone doesn't necessarily improve reliability. A twin engine has twice the probability of an engine failure and engine out failures in twins are more likely to be fatal than single engine failures. In this case, the real improvement in reliability comes from pilot proficiency with single engine operations.

Very interesting post! Many thanks! However your statement "engine out failures in twins are more likely to be fatal than single engine failures" as far as I know is aimed at the private pilot/general aviation sector (apart from the fact that there are no single engine commercial airplanes)?

Three or four hydraulics statement without understanding how the systems are interrelated is an oversimplification. Two controllers can be safer than three depending on how the engineers design for covert failures (an undetected failure). If the probability of covert failures is very small then two controllers will have a higher availability than three controllers, especially if the three are configured in a two-out-of-three voting scheme. This scheme requires that all three be operating to achieve required reliability whereas the one-out-of-two scheme will be almost as "safe" with one failure (give covert failures are nil). This is a very complex and interesting engineering problem, one that I have dealt with in my job as an engineer and there is an engineering field called reliability engineering that works with this body of knowledge full time. The early DC10s had a problem with hydraulics but the MD11 has addressed those failure modes and is considered state-of-the-art "safe."

I understand the re

[downscc 1,327]

The twin vs single definitely applies to light twins, even the cabin class twin I fly has plenty of power on one engine to make single engine operation safer (I've had two engine failures and most twin jet commercial pilots will not have a failure in a lifetime).I do mean the statement that two controllers can be more reliable than three. I don't mean this is always true. I have designed safety shutdown systems for direct fired furnaces in refineries using dual controllers with watchdog timers to eliminate a covert failure of the controller and I can attain higher availability than PLCs with three controllers using two out of three voting. The difference in this case is 1oo2 has lower failure rate because there are fewer components than 2oo3 and the availability for 1oo2 is higher because the 2oo3 scheme requires all three controllers. This can be reversed by adding covert failure detection to the three controllers but for the cost the dual controllers are a better solution. Detecting covert failures is a big deal in reliability engineering.

[Guest Moonraker]

The twin vs single definitely applies to light twins, even the cabin class twin I fly has plenty of power on one engine to make single engine operation safer (I've had two engine failures and most twin jet commercial pilots will not have a failure in a lifetime).I do mean the statement that two controllers can be more reliable than three. I don't mean this is always true. I have designed safety shutdown systems for direct fired furnaces in refineries using dual controllers with watchdog timers to eliminate a covert failure of the controller and I can attain higher availability than PLCs with three controllers using two out of three voting. The difference in this case is 1oo2 has lower failure rate because there are fewer components than 2oo3 and the availability for 1oo2 is higher because the 2oo3 scheme requires all three controllers. This can be reversed by adding covert failure detection to the three controllers but for the cost the dual controllers are a better solution. Detecting covert failures is a big deal in reliability engineering.

Very interesting! You have convinced me that the 2 autopilots/3 hydraulic systems design of the MD11 may be as safe as the 747 design :(However I

[downscc 1,327]

This type of statistic, while interesting, ignores the cause of hull loss. It is pilot error in most cases. However, I don't want to debate who the best operators are, but I have always admired the Boeing engineering solutions over anything else. I thought we were talking about the redundancy and how reliable various schema are in comparison.

[Guest Moonraker]

This type of statistic, while interesting, ignores the cause of hull loss. It is pilot error in most cases.

I agree. That

[stevenp 1]

I agree. That