Boeing B717: why the differential elevator positions when parked? Plus cabin pressure Q

Flew recently (as a passenger) on a Boeing B-717. While waiting to board, I noticed that the left and right elevators were at different positions. Interestingly, one of them appeared to be at maximum upward deflection. Why might that be? If hydraulic pressure fades (because the engines are shut down), wouldn’t both elevators relax downward due to gravity? And why are they able to move independently? Do they do so in flight (as is done on some fighter jets for roll control)?

Second question:
During the descent on that flight, I was watching a plot of cabin pressure versus time on my cell phone. Rather than steadily increasing, the cabin pressure underwent a step change every 5-10 seconds. The steps were rounded (it takes some time to bleed out enough air to reach the new setpoint), but they were distinct. Why might the cabin pressure system operate like this, instead of steadily increasing pressure as the plane descends?

I’ll speculate on your first question but I’m pretty confident in my hypothesis for your second question:

  1. In the absence of hydraulic pressure, both elevators were drooping, but to different degrees, right? I’d guess that the elevator that drooped less had a little more friction in its bearings and/or hydraulic actuator than the droopier elevator did. That may be the case even if one elevator was angled up and the other drooped. Either way, I’d bet the higher-angled elevator had enough stiction not to move once the hydraulic pressure bled down.

  2. Regarding the stepwise rise in cabin pressure, I’d bet you’re seeing the effects of pulse-width modulation (PWM). Cabin pressurization systems use actively-controlled valves, and it’s fairly tricky to control a valve such that it’s held open a certain amount, thereby allowing a particular flow rate.

Flow rate depends on valve opening position, but also air pressure, temperature and flow direction. Also, solenoid valves are likely candidates for this sort of thing, and they’re inherently bad at holding a partially-open position. They’re typically either open or closed. By opening a valve for two seconds and then closing it for eight, you get about 20% of the max flow rate without needing a more-expensive, more-complicated valve that can be held partly open and the attendant flow controller to measure the flow and adjust the valve to hold the desired flow rate.

So, most likely, the environmental control system is opening the relevant valve fully for a moment and then closing it for ten seconds. The finite outflow rate produces the curve you see at the edge of the plot’s steps, but these small, short steps approximate a slow, steady re-pressurization rate that’s friendly to human eardrums.

P.S. From your other posts, Machine Elf, I’m guessing you may already know about proportional valves and PWM. I’m not trying to patronize you; I explained in some detail for anyone without that background.

The “Boeing 717”, nee McDonnell-Douglas MD-95, and the DC-9 series that begat it, has elevators that are aerodynamically actuated. They have a small control tab that is linked to the cockpit columns via a run of cable and drums. This tab, being on the trailing edge of the surface has leverage, and in doing so more or less “flies” the elevator into position. Absent airflow over the surface, the elevators may droop or be deflected upwards by a gust of wind. The control tabs operate in concert, but the surfaces are free to move about independently. There is no hydraulic control for the elevator, except for down only for automatic stall recovery.

On the same series of planes, the ailerons operate in the same manner.