BobDDuck
Island Bus Driver
This is pieced together from my systems knowledge, FAA ADs, letters from the company and discussions with mechanics. While the CRJ hydraulic systems are similar on all 3 (actually, I guess that's 4 now) models this applies specifically to the 200.
The CRJ2 has three independent hydraulic systems, which unlike the creative artists in France are not differentiated by color but rather by number. Systems 1 and 2 are each powered by an engine driven pump and backed up by an AC Motor Pump (ACMP) which operates either on demand by a switch or automatically during critical phases of flight. Systems 1 and 2 power things like the flight controls and spoilers.
System 3 is powered by 2 ACMPs and powers the flight controls as well as the landing gear (which is important later on in this discussion). Systems 2 and 3 share duties for providing pressure for the brakes with system 2 handling the outboard wheels and system 3 handling the inboard wheels. As certified, ACMP 3a runs whenever the airplane is operating off the gate (via a switch on the overhead panel) and ACMP 3b operates on demand via a switch or during critical phases of flight, like the backup pumps for systems 1 and 2.
System demand often comes in large chunks like lots of flight control movement while moving the landing gear. All systems operate at about 3000psi and to ensure uninterrupted pressure system 3 (as well as system 2, but that isn't as important to this discussion) has an accumulator that stores pressure and puts it in to the system as needed. This accumulator pressure is also used to hold the parking brake on the inner wheels after the aircraft is shut down and no pumps are running. The pressure eventually bleeds out but the hope is that by then you have chocks in place.
Over the last several years there have been four documented cases (none at the airline I work for) of the end caps of the 3 system accumulator catastrophically failing. The problem is that due to the placement of the accumulator, when the cap blows off the end, it just so happens that in it's trajactory hit pressure lines for all three hydraulic systems as well as several other essential cables, wires, ducts and assorted equipment. Fortunately all four incidents occurred at the gate and while extensive damage was done to the airframe there was never an risk of a crash.
After looking into the problem Transport Canada (because the aircraft is certified there) came up with their equivalent of an AD.Operators could either install a different type of accumulator or could remove the accumulator all together and run ACMP 3b at all times in addition to ACMP 3a to ensure enough pressure was in the system. Several US operators started in on this compliance even before the FAA issued their own AD. One operator removed all of the accumulators while another started replacing accumulators with a different brand. Unfortuantly for this second operator when the FAA came out with their AD it did not allow for an alternate brand, but rather only the complete removal of the accumulator.
As required, we complied with the AD here and removed both the System 2 and 3 accumulators and started running the 3b pump at all times from power on in the morning to power off at night. We'd been warned by the operator the did this as soon as Transport Canada issued their AD that this would lead to a much higher failure rate of 3b pumps due to their constant use and hence we ensured we had a good supply of spares.
(An interesting side note: during a complete power failure the ADG powers pump 3b directly. Of course it is very unlikely that you'd ever end up with EMER POWER ONLY but if it were to happen, wouldn't it just suck if that was the day the overworked 3b pump decided to die?)
Anyhow, we switched to this new form of operation and despite some grumbles about change and what not it seemed to be working out.
Fast forward about 3 months and it has become apparent that the number of gear failures (either failing to retract or failing to extend) has skyrocketed, both at this company and other other CRJ 200 operators. After much head scratching and pondering our Maintenance and Engineering department (as well as other operator's M&Es) have discovered that even with 3b and 3a running simultaneously, there still isn't always enough damping in the system to steadily move the gear and it, for lack of better word, "bounces" slightly and sets of all kinds of sensors that cause the system to stop moving, leaving the gear in a position that doesn't match the handle. Emergencies and lots of required maintenance inspections (which require towing the airplane off to a hangar to swing the gear to make sure it actually does work) ensue.
The best part is absolutely nothing can be done about this right now. Until the FAA allows for an alternate method of compliance with this AD operators are stuck doing what they are doing right now. So we will continue to have gear failures in the future which require return to fields and emergencies declared as well as inspections. Additionally, as has been demonstrated a few operators recently, every one in a while this has lead to a gear not being able to be extended, even manually and an aircraft incident.
Eventually there will be an AMOC issued which will allow for either the movement of the accumulator to another location or the installation of a different brand accumulator. But for now, every time a crew pulls up the gear and drops it down, they are going to have to wonder if it's actually going to work.
The CRJ2 has three independent hydraulic systems, which unlike the creative artists in France are not differentiated by color but rather by number. Systems 1 and 2 are each powered by an engine driven pump and backed up by an AC Motor Pump (ACMP) which operates either on demand by a switch or automatically during critical phases of flight. Systems 1 and 2 power things like the flight controls and spoilers.
System 3 is powered by 2 ACMPs and powers the flight controls as well as the landing gear (which is important later on in this discussion). Systems 2 and 3 share duties for providing pressure for the brakes with system 2 handling the outboard wheels and system 3 handling the inboard wheels. As certified, ACMP 3a runs whenever the airplane is operating off the gate (via a switch on the overhead panel) and ACMP 3b operates on demand via a switch or during critical phases of flight, like the backup pumps for systems 1 and 2.
System demand often comes in large chunks like lots of flight control movement while moving the landing gear. All systems operate at about 3000psi and to ensure uninterrupted pressure system 3 (as well as system 2, but that isn't as important to this discussion) has an accumulator that stores pressure and puts it in to the system as needed. This accumulator pressure is also used to hold the parking brake on the inner wheels after the aircraft is shut down and no pumps are running. The pressure eventually bleeds out but the hope is that by then you have chocks in place.
Over the last several years there have been four documented cases (none at the airline I work for) of the end caps of the 3 system accumulator catastrophically failing. The problem is that due to the placement of the accumulator, when the cap blows off the end, it just so happens that in it's trajactory hit pressure lines for all three hydraulic systems as well as several other essential cables, wires, ducts and assorted equipment. Fortunately all four incidents occurred at the gate and while extensive damage was done to the airframe there was never an risk of a crash.
After looking into the problem Transport Canada (because the aircraft is certified there) came up with their equivalent of an AD.Operators could either install a different type of accumulator or could remove the accumulator all together and run ACMP 3b at all times in addition to ACMP 3a to ensure enough pressure was in the system. Several US operators started in on this compliance even before the FAA issued their own AD. One operator removed all of the accumulators while another started replacing accumulators with a different brand. Unfortuantly for this second operator when the FAA came out with their AD it did not allow for an alternate brand, but rather only the complete removal of the accumulator.
As required, we complied with the AD here and removed both the System 2 and 3 accumulators and started running the 3b pump at all times from power on in the morning to power off at night. We'd been warned by the operator the did this as soon as Transport Canada issued their AD that this would lead to a much higher failure rate of 3b pumps due to their constant use and hence we ensured we had a good supply of spares.
(An interesting side note: during a complete power failure the ADG powers pump 3b directly. Of course it is very unlikely that you'd ever end up with EMER POWER ONLY but if it were to happen, wouldn't it just suck if that was the day the overworked 3b pump decided to die?)
Anyhow, we switched to this new form of operation and despite some grumbles about change and what not it seemed to be working out.
Fast forward about 3 months and it has become apparent that the number of gear failures (either failing to retract or failing to extend) has skyrocketed, both at this company and other other CRJ 200 operators. After much head scratching and pondering our Maintenance and Engineering department (as well as other operator's M&Es) have discovered that even with 3b and 3a running simultaneously, there still isn't always enough damping in the system to steadily move the gear and it, for lack of better word, "bounces" slightly and sets of all kinds of sensors that cause the system to stop moving, leaving the gear in a position that doesn't match the handle. Emergencies and lots of required maintenance inspections (which require towing the airplane off to a hangar to swing the gear to make sure it actually does work) ensue.
The best part is absolutely nothing can be done about this right now. Until the FAA allows for an alternate method of compliance with this AD operators are stuck doing what they are doing right now. So we will continue to have gear failures in the future which require return to fields and emergencies declared as well as inspections. Additionally, as has been demonstrated a few operators recently, every one in a while this has lead to a gear not being able to be extended, even manually and an aircraft incident.
Eventually there will be an AMOC issued which will allow for either the movement of the accumulator to another location or the installation of a different brand accumulator. But for now, every time a crew pulls up the gear and drops it down, they are going to have to wonder if it's actually going to work.
