Split Pressure IndicationWith identical engine bleed air systems on either side of the normally closed isolation valve, it isreasonable to expect essentially the same pressure indications on both sides of the isolation valve,when operating at the same EPR. However, different pressure indications are often noticed duringhigh altitude cruise, at the end of cruise or while cruising at a low gross weight. A split pressureindication during above mentioned operating conditions is not a positive indication of a malfunction.It can occur, in a normal system, due to one or more of the following:
• Tolerance of system control valves. The PRSOV and the high valve each have a pressureregulation tolerance of ± 5 psig.• Tolerance of the duct pressure transmitter and gauge indication system.• Engine bleed pressure variations between engines due to deterioration or variations in enginebuild tolerances, etc.• One engine bleed air system operating on the low and the other on the high stage bleeddue to tolerances in the switching pressures.The first three items above, can cause large differential pressure indications if the controlcomponents of the two engine bleed air systems are operating at, or close to, the extremes of theallowable tolerance band. These items cause split pressure indications when the system valvesare in control, that is, during moderate to high power operations. During low power operations, when the control valves are essentially wide open, the pressure differential generally reduces. Aconsistent differential pressure greater than 10 psig during climb power, though not an indication ofa malfunction, should be noted and may be cause for maintenance action.The last item above, can cause differential pressure indications of 20-30 psig. These generallyoccur at high altitude and a low engine thrust when the engines are operating near the switchingpressure threshold or switchover EPRs.From the above discussion, it would be apparent that split pressure observation is not a positiveindication of a fault or premature selection of the high stage bleed air. Switching from low tohigh stage occurs with decreasing EPR and is accompanied by a sudden increase in manifoldpressure. Switching from high to low occurs with increasing EPR and is accompanied by adecrease in manifold pressure.