In the 1960s and 70s, it was not uncommon for commercial airplanes to have “load sharing,” electrical systems. In other words, all generators could be “paralleled” onto all electrical busses simultaneously. This could be problematic. In the event of a generator or engine failure, the remaining generators might be overloaded trying to supply the load. A primary responsibility of the Flight Engineer was to manage such a situation. Most generators could handle a temporary overload, say perhaps 5 seconds. Within those 5 seconds, the Flight Engineer had to reduce the electrical power that the airplane was using and thereby protect the essential items. Management of “essential power” was implicated as a probable or contributing cause in at least on 727 accident (UAL 266). Essential power fed the captains instruments.

With the introduction of the DC-9, a different type of electrical system was utilized. This type of electrical system may have been used on other airplanes, but the DC-9 was widely sold. The DC-9 was also one of the early 2 crew airplanes, no Flight Engineer. This system has been called a “split bus” system and the main operating principle or rule is that only one generator can power an electrical bus at one time. A single generator can power multiple busses, but a bus can only have one generator connected at a time. The 737 followed suit with the same design, and almost everything built since then has a similar system.

Back to load sharing. On the prior airplanes, the actual process of “paralleling” the generators could be problematic. There are technical things beyond the scope of this post that I will mention but not describe: The phases of 3 phase alternating current have to be “in phase.” There was an art to managing these systems; some airplanes (727) had synchronizing lights (I wish I could figure out ho to append a picture, they are out there) to show the variations in the electrical power that one could look at when attempting to ascertain the exact moment to bring a generator on line.

Let’s get to the fun part. Bringing a generator on line in the L1011 was a snap. The airplane would monitor all the parameters and bring the generator on-line when it was appropriate if the switchlight (L1011 switchlights could be another topic) was latched in. If the airplane was carrying a load of 90kva, each of the three main engine generators would each be carrying 30kva. The unique part on the L1011 is that it would parallel the APU with the engine generators! It would do this inflight or on the ground. All the generators were identical; they were the same part number and were rated at 90kva. Thus, you could say that the L1011 had 4 main electrical power sources. Many people have commented that the 747 and DC10 only had 3 hydraulic systems whereas the L1011 had 4. Not a lot of people know that the APU on the L1011 would parallel with the engine generators and therefore essentially be a 4th power source that was equivalent to the main engine generators. The airplane was not operated routinely in this fashion. There was no need. Perhaps this is another example of the “over the top” engineering that was designed into the L1011. The L1011 also had other features that it would execute automatically, not dependent upon the Flight Engineer. Load shedding for example, which is now standard on modern airplanes.

I’ve looked and have so far been unable to find the altitude capability of the L1011 APU for supplying electrical power. On modern twins, the APU will supply electrical power up to the altitude capability of the airplane. They also have modifications to ensure that they start inflight. I’m not sure how reliable the APU on the L1011 was on starts at altitude. Monitoring inflight restart reliability didn’t really become a thing until the advent of ETOPS. But the paralleling APU generator on the L1011 was unique in my experience. If anyone knows any other airplane that would do this, by all means let us know. Its unfortunate that some of the folks who you to post on this forum regarding the L1011 are no longer with us.

Very interesting, another remarkable and highly advanced feature of the superb L1011


Incidentally while the DC10 only had three hydraulic systems the 747 did have four


I couldn’t agree more with your last two sentences

Max Q wrote:

Very interesting, another remarkable and highly advanced feature of the superb L1011


Incidentally while the DC10 only had three hydraulic systems the 747 did have four


I couldn’t agree more with your last two sentences

Oops my bad re 747, thanks for the correction.

SteelChair wrote:

On the prior airplanes, the actual process of “paralleling” the generators could be problematic. There are technical things beyond the scope of this post that I will mention but not describe: The phases of 3 phase alternating current have to be “in phase.” There was an art to managing these systems; some airplanes (727) had synchronizing lights (I wish I could figure out ho to append a picture, they are out there) to show the variations in the electrical power that one could look at when attempting to ascertain the exact moment to bring a generator on line..

The 727 would auto-parallel if the generator breaker switch was used. The synchronizing lights would only be needed when using the bus tie switches to bring power on to the bus. I’m not sure why you would use the bus tie switches to bring power on the bus, but the procedure was so drilled into our heads during training that most people used the sync lights anyway when using the generator switches (myself included).



A decent explanation of the paralleling procedure is about 1/3 of the way down the page: https://www.boeing-727.com/Data/systems/infoelect.html

DL_Mech wrote:

SteelChair wrote:

On the prior airplanes, the actual process of “paralleling” the generators could be problematic. There are technical things beyond the scope of this post that I will mention but not describe: The phases of 3 phase alternating current have to be “in phase.” There was an art to managing these systems; some airplanes (727) had synchronizing lights (I wish I could figure out ho to append a picture, they are out there) to show the variations in the electrical power that one could look at when attempting to ascertain the exact moment to bring a generator on line..

The 727 would auto-parallel if the generator breaker switch was used. The synchronizing lights would only be needed when using the bus tie switches to bring power on to the bus. I’m not sure why you would use the bus tie switches to bring power on the bus, but the procedure was so drilled into our heads during training that most people used the sync lights anyway when using the generator switches (myself included).

A decent explanation of the paralleling procedure is about 1/3 of the way down the page: https://www.boeing-727.com/Data/systems/infoelect.html

I assume you are talking about auto paralleing thr main engine generators....not the APU.

SteelChair wrote:

I assume you are talking about auto paralleing thr main engine generators....not the APU.

Yes.

DL_Mech wrote:

SteelChair wrote:

On the prior airplanes, the actual process of “paralleling” the generators could be problematic. There are technical things beyond the scope of this post that I will mention but not describe: The phases of 3 phase alternating current have to be “in phase.” There was an art to managing these systems; some airplanes (727) had synchronizing lights (I wish I could figure out ho to append a picture, they are out there) to show the variations in the electrical power that one could look at when attempting to ascertain the exact moment to bring a generator on line..

The 727 would auto-parallel if the generator breaker switch was used. The synchronizing lights would only be needed when using the bus tie switches to bring power on to the bus. I’m not sure why you would use the bus tie switches to bring power on the bus, but the procedure was so drilled into our heads during training that most people used the sync lights anyway when using the generator switches (myself included).

A decent explanation of the paralleling procedure is about 1/3 of the way down the page: https://www.boeing-727.com/Data/systems/infoelect.html

We had the same procedure (paralleling with the GBs while synchronizing with the synch lights


I’m not sure either why you’d use the BTBs to bring power on to the bus but we were taught to use extra care synchronizing if we ever had to do that

DL_Mech wrote:

SteelChair wrote:

I assume you are talking about auto paralleing thr main engine generators....not the APU.

Yes.

You seem to have a lot of systems knowledge. Do you know of any airplane that would parallel the APU?

If the BTB tripped with the GB closed, you could have to manually parallel or, just trip that GB, close the BTB and auto-parallel as you would during engine start. IIRC, our instructor offered that if your probation and ran into this problem.

Or, smelter yet, just leave the BTB open, as there must have been a good reason for it to trip.

Since my first post was so verbose, there were other items I didn't add. "No break power transfer," has become fairly common on the latest generations of airplanes. On the older jets, when you switched electrical power from ground power to the APU, the lights flickered. (On the 727 there was also a loud clunk in the cockpit, the relays were large and loud.) On the L1011, if you timed your switchover from ground power to APU, the lights would not flicker. As I recall, you looked at the ammeter for the APU battery, and when it cycled as the battery charger kicked in momentarily, you could hit the switch for the APU and it would come on seamlessly. If I got that incorrect, I'm sure someone will correct me....the thing I remember is that there was a way to do it.

Having operated the 747 classic for many years, the following about the 747 electrical system :
After starting the engines, all CSD and Gen lights were checked out, there after the channels were paralled with BTB's.
The 747 has 4 engine AC generators that could be paralled by closing the Bus Tie breakers (BTB's) and the split system Breaker (SSB). When closing a BTB switch one of the Generators received an electric signal to slow down the RPM (In the CSD) , to sycronice the phases, thereafter the BTB (relay) kicked in.
The APU generator(s) or ground electric was auto disconnected when switching the engine generators via the BTB's on line.
When switching the source from APU Generators or ground electric to the engine generators (or the other way around) some light flickering was observed. (APU Generators had no CSD !)
Switching an engine generator on or off was not visible (no light flickering) and also in certain fault cases an automatic BTB re-closure could happen leaving the relevant AC BUS powered , but than the gen open light illumunated.

The APU generators were not certified for in flight use, However the APU could be used in flight (during T/O) for supplying pneumatic air to ACM #2, during an all packs off T/O, very common with the early JT9D-3AW, -7(A)W and -7FW engines (waterinjection , packs off T/O ).
Later, the APU inlet was modified (for fuel saving) and the APU was only certified for on ground use.

747 :
four (4) AC Engine Generators
four (4) hydraulic systems
1 or two APU generators, only for on ground use, on combi aircraft the second generator was normally used for the maindeck cargo loading handling
All AC generators are identical, however due forced cooling the APU generators were rated at 90KVA , the engine generators at 60KVA.

GalaxyFlyer wrote:

If the BTB tripped with the GB closed, you could have to manually parallel or, just trip that GB, close the BTB and auto-parallel as you would during engine start. IIRC, our instructor offered that if your probation and ran into this problem.

Or, smelter yet, just leave the BTB open, as there must have been a good reason for it to trip.

That makes sense

I think I may have done that once or twice (paralleled) with the BTB but it was rare

747classic wrote:

Having operated the 747 classic for many years, the following about the 747 electrical system :
After starting the engines, all CSD and Gen lights were checked out, there after the channels were paralled with BTB's.
The 747 has 4 engine AC generators that could be paralled by closing the Bus Tie breakers (BTB's) and the split system Breaker (SSB). When closing a BTB switch one of the Generators received an electric signal to slow down the RPM (In the CSD) , to sycronice the phases, thereafter the BTB (relay) kicked in.
The APU generator(s) or ground electric was auto disconnected when switching the engine generators via the BTB's on line.
When switching the source from APU Generators or ground electric to the engine generators (or the other way around) some light flickering was observed. (APU Generators had no CSD !)
Switching an engine generator on or off was not visible (no light flickering) and also in certain fault cases an automatic BTB re-closure could happen leaving the relevant AC BUS powered , but than the gen open light illumunated.

The APU generators were not certified for in flight use, However the APU could be used in flight (during T/O) for supplying pneumatic air to ACM #2, during an all packs off T/O, very common with the early JT9D-3AW, -7(A)W and -7FW engines (waterinjection , packs off T/O ).
Later, the APU inlet was modified (for fuel saving) and the APU was only certified for on ground use.

747 :
four (4) AC Engine Generators
four (4) hydraulic systems
1 or two APU generators, only for on ground use, on combi aircraft the second generator was normally used for the maindeck cargo loading handling
All AC generators are identical, however due forced cooling the APU generators were rated at 90KVA , the engine generators at 60KVA.

Very interesting, not heard of a split system breaker before, what is its function ?

SSB is used to split / connect the left sync bus and right synch bus (creating the "full" tie bus)
Closed with all 4 Eng generators are paralled durng normal ops
Open when APU Gen #1 and APU #2 are powering the system (APU Gen have no CSD and can not be parelled)
The same is valid if both Ext pwr #1 and Ext pwr #2 are powering the system

Sharp observers may note that on the picture of the 747 Flight Engineer panel there is a switch labeled "essential power," and that that switch has 4 positions. Gen1, 2, 3, and normal. Thus, it would appear that selection of what powered the essential bus was a totally manual selection. Does anyone know, if in the event of the failure of a generator, the 747 would select another bus?

On the L1011, this same switch had 6 positions. It had manual positions and auto positions. So for example if the switch was in 3/2 auto , and the number 3 system was powering essential and it failed, the airplane would switch what was powering essential to number 2 automatically (if the switch was in an auto 3/2 position). And it would do it faster than thr FE could even diagnose what had happened, let alone take action. I'm not sure what the normal position for the switch to be in was..

Does anyone know how essential was powered on the DC10?

SteelChair wrote:

Sharp observers may note that on the picture of the 747 Flight Engineer panel there is a switch labeled "essential power," and that that switch has 4 positions. Gen1, 2, 3, and normal. Thus, it would appear that selection of what powered the essential bus was a totally manual selection. Does anyone know, if in the event of the failure of a generator, the 747 would select another bus?

prerequisites : all engine generators paralled (normal flight) :

Short answer : NO

The normal position means that AC bus #4 is powering the essential power bus.

If Gen #4 fails the #4 generator breaker will open, but the AC bus #4 will remain powered via BTB #4 and the tie bus, powered by generators 1,2 and 3

If one of the other generators (1,2 or 3) fails, also no essential pwr switching is needed.

Only when AC bus #4 fails ("essential bus off" light will illum ), the F/E has to manually switch the essential pwr bus directly to an operating generator (pwr drawn before the Gen breaker !!)

747classic wrote:

SteelChair wrote:

Sharp observers may note that on the picture of the 747 Flight Engineer panel there is a switch labeled "essential power," and that that switch has 4 positions. Gen1, 2, 3, and normal. Thus, it would appear that selection of what powered the essential bus was a totally manual selection. Does anyone know, if in the event of the failure of a generator, the 747 would select another bus?

prerequisites : all engine generators paralled (normal flight) :

Short answer : NO

The normal position means that AC bus #4 is powering the essential power bus.

If Gen #4 fails the #4 generator breaker will open, but the AC bus #4 will remain powered via BTB #4 and the tie bus, powered by generators 1,2 and 3

If one of the other generators (1,2 or 3) fails, also no essential pwr switching is needed.

Only when AC bus #4 fails ("essential bus off" light will illum ), the F/E has to manually switch the essential pwr bus directly to an operating generator (pwr drawn before the Gen breaker !!)

Thanks for that. I guess the engineers of the 1960s and early 70s at Boeing and Douglas just didn't trust automatic features. Or they just didn't want to put the time and effort into engineering the advanced automated features. Lockheed was so far ahead imho.

IMHO, the 747 was a straight fwd design, with a lot of redundancy built in.
All systems were not very complicated, as at the DC10 with all reversible and non reversible pumps, etc.
I completed the full F/E DC10 course and had my first training flight AMS-ANC-HND . While in Tokyo all DC 10 's were grounded (after the Chicago crash) , so after returning to AMS as a passenger, I was transferred to the 747-200 to become the first direct entry F/E. So I can compare the two aircraft systems.

The L1011 was indeed a technical masterpiece, however with one major design failure :

The failure to cater for the possibility of higher operating weights for more range/ passengers, due main landing gear limitations.(no space for triple axle gears )
Even the DC-10 was designed from the start with the possibility of installing a center gear., the 747 had from the start four main gear trucks.
To cater for more range, the L1011 had to be shortend
A design was made with two three axle trucks (L1011-8), but a lot of re-design effort was needed to create enough space,, conseq the price was far too high compared to the DC10-30/40, Lockheed could not find a launch custome. Together with the financial difficulties of both Lockheed and RR the program was finally terminated after only 250 aircraft.

Roger the center gear issue. That and the financial difficulties of RR/no second engine option on the L1011 have long been described as major limitations of the program, along with not having any other airplanes in their product line..

However, my creation of these systems threads was not to dwell upon the well known failures of the business plan, but rather to specify some of the technical achievements. IMHO a lot of people are aware that the L1011 was a technical masterpiece but fail to appreciate the extent of that technical superiority. Lots of people cite DLC but don't realize many of the other systems and capabilities. I've only pointed out a few.

Perhaps Lockheed management is to be faulted because they designed so much into this airplane at the very nadir of technical innovation in commercial aviation. By the 80s, airplane design was going the other way, more simplicity but extreme levels of reliability designed into those systems. I guess the same could be said of the 747, Boeing didn't foresee that quads would have a limited lifespan, no one could foresee the extreme levels of engine reliability. In the same way, I suppose it could be argued that today, many refuse to see that aircrew will be limited/further restricted/done away with over the next 50 years.

747classic wrote:

IMHO, the 747 was a straight fwd design, with a lot of redundancy built in.
All systems were not very complicated, as at the DC10 with all reversible and non reversible pumps, etc.
I completed the full F/E DC10 course and had my first training flight AMS-ANC-HND . While in Tokyo all DC 10 's were grounded (after the Chicago crash) , so after returning to AMS as a passenger, I was transferred to the 747-200 to become the first direct entry F/E. So I can compare the two aircraft systems.

The L1011 was indeed a technical masterpiece, however with one major design failure :

The failure to cater for the possibility of higher operating weights for more range/ passengers, due main landing gear limitations.(no space for triple axle gears )
Even the DC-10 was designed from the start with the possibility of installing a center gear., the 747 had from the start four main gear trucks.
To cater for more range, the L1011 had to be shortend
A design was made with two three axle trucks (L1011-8), but a lot of re-design effort was needed to create enough space,, conseq the price was far too high compared to the DC10-30/40, Lockheed could not find a launch custome. Together with the financial difficulties of both Lockheed and RR the program was finally terminated after only 250 aircraft.

And yet we have the A350-900 at up to 280,000kg using only two 4-wheel trucks, that's 40,000kg more than the L1011 topped out at. I guess the A350 wheels are spaced out a bit more and the gear is certainly much beefier, but still interesting point. I think if there was another engine option, it may have garnered enough interest to develop it further. Such a pity..

I wonder if using all the advanced technology that's been mentioned should have been used to make a 2-man flight deck. Having that in the early 70s would've been a big selling point over a DC-10. As it stands all the advancements the L1011 had increased purchase price but didn't lead to any real advantages in performance/efficiency. Meanwhile, the RB211 issues and the limitations of the landing gear only made things worse. It took another 10 years with the 767 (followed shortly after by the A310 and A306) for a widebody with a 2-man cockpit.

The L1011 did have some problems with rear spar cracking, along with some other large Lockheed airplanes. I'm not sure that adding a six wheel or a widely spaced 4 wheel gear would have worked without major spar work.

Regarding the two man cockpit, getting the 767 and A300 certified were huge hurdles. Some early A300s did have Flight Engineers and the 767 had a small panel, the space was "staked out" in the cockpit in case the regulators wouldn't come around to approving 2 crew. Eventually they did and the "unsafe" two man airplanes have been several orders of magnitude safer than the 3 crew airplanes. I just don't think that the industry was ready for a two man flight deck on the L1011 in 1970. If anyone could have done it, it would have been Lockheed.

747classic wrote:

SSB is used to split / connect the left sync bus and right synch bus (creating the "full" tie bus)
Closed with all 4 Eng generators are paralled durng normal ops
Open when APU Gen #1 and APU #2 are powering the system (APU Gen have no CSD and can not be parelled)
The same is valid if both Ext pwr #1 and Ext pwr #2 are powering the system

Very interesting and enlightening as usual, there’s not that much difference between the basic designs of the electrical panels on the 707, 727 and 747


I read recently that Lufthansa wanted Boeing to build the 747 as a two pilot aircraft, automating the FE function as they had already done on the preceding 737-100


While technically possible the airlines ( not to mention aircrew unions) weren’t ready for such a step in such a large significantly more complex aircraft that would be operated in remote areas around the world and having a professional FE was always a big advantage in that aircraft



It took until 1989 when the state of the art advanced sufficiently to build a two pilot aircraft with the 747-400


Lockheed did test an L1011 cockpit with EFIS but I don’t believe they even had a concept for two pilot operation

SteelChair wrote:

The L1011 did have some problems with rear spar cracking, along with some other large Lockheed airplanes. I'm not sure that adding a six wheel or a widely spaced 4 wheel gear would have worked without major spar work.

Regarding the two man cockpit, getting the 767 and A300 certified were huge hurdles. Some early A300s did have Flight Engineers and the 767 had a small panel, the space was "staked out" in the cockpit in case the regulators wouldn't come around to approving 2 crew. Eventually they did and the "unsafe" two man airplanes have been several orders of magnitude safer than the 3 crew airplanes. I just don't think that the industry was ready for a two man flight deck on the L1011 in 1970. If anyone could have done it, it would have been Lockheed.

I still think that a modern aircraft with a three crew cockpit would be safer than a modern two crew aircraft. The increase in safety was not because of the reduction from 3 to 2 crew !!!
A third pair of eyes, behind the pilots, that can compare the screens/ situation in front of them in an emergency wouild be a lot safer, see a lot of recent accidents.
I still consider the optimum crew composition in an emergency - PF only flying/navigate the aircraft, the two other crew deal with the emergency, one is trouble shooting/ switching the other is checking the non-nomal procedures. During that proces the PF will be regulary informed about the progress.

From memory :
The Qantas A380 accident was managed by a 3+ crew, a 2 man crew would have been overwhelmed by the multiple conflicting warnings. Also the automation was not working as it should be.
The same is valid for the KLM 747-400 Anchorage accident. Luckely an instructor (captain, just transferred from the 747 classic- I knew him very well) was seated behind the pilots to manage the situation, otherwise the outcome could have been very different. Here also the automation partly failed due multiple failures.

But all above has nothing to do with the L1011 electrical system, so back to the subject.

747classic wrote:

SteelChair wrote:

The L1011 did have some problems with rear spar cracking, along with some other large Lockheed airplanes. I'm not sure that adding a six wheel or a widely spaced 4 wheel gear would have worked without major spar work.

Regarding the two man cockpit, getting the 767 and A300 certified were huge hurdles. Some early A300s did have Flight Engineers and the 767 had a small panel, the space was "staked out" in the cockpit in case the regulators wouldn't come around to approving 2 crew. Eventually they did and the "unsafe" two man airplanes have been several orders of magnitude safer than the 3 crew airplanes. I just don't think that the industry was ready for a two man flight deck on the L1011 in 1970. If anyone could have done it, it would have been Lockheed.

I still think that a modern aircraft with a three crew cockpit would be safer than a modern two crew aircraft. The increase in safety was not because of the reduction from 3 to 2 crew !!!
A third pair of eyes, behind the pilots, that can compare the screens/ situation in front of them in an emergency wouild be a lot safer, see a lot of recent accidents.
I still consider the optimum crew composition in an emergency - PF only flying/navigate the aircraft, the two other crew deal with the emergency, one is trouble shooting/ switching the other is checking the non-nomal procedures. During that proces the PF will be regulary informed about the progress.

From memory :
The Qantas A380 accident was managed by a 3+ crew, a 2 man crew would have been overwhelmed by the multiple conflicting warnings. Also the automation was not working as it should be.
The same is valid for the KLM 747-400 Anchorage accident. Luckely an instructor (captain, just transferred from the 747 classic- I knew him very well) was seated behind the pilots to manage the situation, otherwise the outcome could have been very different. Here also the automation partly failed due multiple failures.

But all above has nothing to do with the L1011 electrical system, so back to the subject.

Agree, having a professional flight engineer in the cockpit, especially when things go wrong is invaluable