So I had an IRS fail. I went through the procedures and I understand the redundancy on the 73 with two IRS and two FMC's. For the life of me I am unable to to get one IRS to command one FMC to Allow Auto Flight reingagement.Someone correct me if I am wrong, but shouldn't one functioning and aligned IRS be able to satisfy the FMC to allow the 73 ng to operate on limited autoflight?So at cruise, I lost the left IRS. At that moment I lost the left PFD and ND. This should happen, since the captains side is being fed by the left FMC, which is also being given data from the left IRS. Then I switched the IRS source switch to, "Both on right", which should send all the FMC's data from the functional right IRS. This worked and I got my captain side PFD and ND back. Now here is the problem. As soon as I switched I was unable to get my FD's back. I recycled the Flight Director switches, but was unable to get the FD on either the capt or FO side. Unfortunately, without the Flight Director being active, I was unable to couple the Autoflight system, which meant I had to continue the flight flying manually for the next 2 hours, which was ok, but without another crewmember, executing flows and just changing frequencies was a bear and almost alway resulted in a slight climb or descent when I got back to flying the plane. Anyways...Shouldn't one functional IRS be enough to still allow for Flight directors and also autoflight?JB

Erm no...Each FCC receives longitudinal and yaw information (displacement and acceleration) from the onside IRU and lateral displacement and acceleration information from the offside IRU.

FMC is not the core of the airplane, a 737 will fly pretty well if FMC has failed, autopilot will still avaiable in all not- fmc related modes.Different for IRS as the IRS tells the autopilot where the aircraft is and how it is moving. Loss of IRS data will disconnect AP as you lose the most important info for it.You can still use the other one as dual IRS and autopilot are present on board.For autopilot engagement 2 most important things are needed:Hydraulic pressure and IRS functioning. The first is needed to move, the second is needed to know how, where, and when to move.If you lose one of them, AP will not engage or if engged will disconnect.