I'm trying the RNAV(RNP) Y approach to runway 31L at KPSP. VNAV kind of works up until the point where the synthetic glideslope should be intercepted, provided one lowers the target altitude setting to below the runway altitude. Most aircraft permit the target altitude to be set to the glideslope intercept altitude for this type of approach. Is VNAV actually the wrong way to fly a VNAV(RNP) approach? How does one select other approach modes?