Dual hydraulic failure and speed increments

[galleypower]

Lets assume your aircraft has two hydraulic systems. Under normal conditions, the flight controls are powered by these systems. If both systems fail, a manual reversion mode is available (e.g. Embraer 145). For approach the pilot has to add +30 knots on the approach speed. What is the reason for such an increment? Could there be any other reason apart from an improved control-ability?

[Mad (Flt) Scientist]

Improved controllability would be the normal reason for adding such an increment.

Something to watch out for, though, is that the increment may also be covering for use of an alternate landing configuration. For example, say your usual landing configuration has a given Vref. It may have been found during flight testing that the best option for the failure was to land at some other flap setting - perhaps a takeoff setting - which in itself requires a certain increment over the normal, landing, Vref to maintain margins to stall speed etc. That might be half of your +30. The other half is then to get a high enough dynamic pressure to give good handling.

In fact, it may be that even had you stayed in the landing config, you'd need to end up at about the same actual speed, but in terms of accounting for the speeds, strictly there is part of the adder now associated with the recommended flaps, not with the failure directly. (Of course, it's all ultimately due to the failure).

I'd expect the OEM will have tried very hard to keep the adder as low as they could, especially for such a case where brake effectiveness may be compromised as well.

[galleypower]

Many thanks for your reply. That all makes sense.