What prevents the extension of the nose gear during an alternate gear extension procedure?

The correct answer regarding what prevents the extension of the nose gear during an alternate gear extension procedure is the malfunctioning of hydraulic system 4. In the CRJ550, the nose gear is primarily operated by the hydraulic systems, and specifically, system 4 is responsible for providing hydraulic pressure necessary for normal operations, including gear extension.

In the case of a malfunction in system 4, this would inhibit the nose gear from extending, as the alternate gear extension relies on the ability to activate mechanisms that depend on that hydraulic pressure. The alternate gear extension procedure is designed for situations where normal hydraulic operation is compromised, but if system 4 is malfunctioning, the procedure cannot successfully deploy the nose gear.

Understanding the role of the other hydraulic systems is also important, as the alternate procedure has contingencies but still requires certain hydraulic systems to function. Systems 2 and 3 have roles in powering other components, not directly in the nose gear operation during alternate extension. An electrical failure specifically related to the nose gear would not typically prevent extension; rather, it could lead to issues indicating the position, but would not inhibit the mechanical operation itself.