Originally Posted by
TheStranger
Thanks for the responses.
Dog,
The manual doesn't state anything more in depth than what I mentioned. If you want to check it out, the Seminole POH PDF is available as a top result in a google search. On the Seminole, the static port is located out on the wing on the Pitot head, so I doubt if warming the cabin would affect it.
JNB,
The go-to systems guy. That makes sense to me. Do you have a resource that could confirm that or are you speaking from your experience?
BTW - you left me hanging with a Vmc thread a while back, guess you were tired of the topic haha - no worries!
I didn't think you were hanging in the Vmc thread, I thought you answered your own questions and a good understanding seemed to develop from it. I didn't think adding anything at that point would be helpful
I may not have seen your last post or last bit either just due to a trip or something like that. Propellor controls to takeoff position is in 23.149, that's where the resulting "windmilling prop" comes from (after failure). Hopefully you passed that with flying colors!
It depends on how far down the rabbit hole you want to go for the static port. In my experience, those are the reasons (calibrated for the most likely scenario) for it. What i mean by this is you would have to probably call the Aircraft Certification Office and the Manufacturer's engineers to know for sure, but:
§23.1325 Static pressure system.
(a) Each instrument provided with static pressure case connections must be so vented that the influence of airplane speed, the opening and closing of windows, airflow variations, moisture, or other foreign matter will least affect the accuracy of the instruments except as noted in paragraph (b)(3) of this section.
(b) If a static pressure system is necessary for the functioning of instruments, systems, or devices, it must comply with the provisions of paragraphs (b)(1) through (3) of this section.
(1) The design and installation of a static pressure system must be such that—
(i) Positive drainage of moisture is provided;
(ii) Chafing of the tubing, and excessive distortion or restriction at bends in the tubing, is avoided; and
(iii) The materials used are durable, suitable for the purpose intended, and protected against corrosion.
(2) A proof test must be conducted to demonstrate the integrity of the static pressure system in the following manner:
(i) Unpressurized airplanes. Evacuate the static pressure system to a pressure differential of approximately 1 inch of mercury or to a reading on the altimeter, 1,000 feet above the aircraft elevation at the time of the test. Without additional pumping for a period of 1 minute, the loss of indicated altitude must not exceed 100 feet on the altimeter.
(ii) Pressurized airplanes. Evacuate the static pressure system until a pressure differential equivalent to the maximum cabin pressure differential for which the airplane is type certificated is achieved. Without additional pumping for a period of 1 minute, the loss of indicated altitude must not exceed 2 percent of the equivalent altitude of the maximum cabin differential pressure or 100 feet, whichever is greater.
(3) If a static pressure system is provided for any instrument, device, or system required by the operating rules of this chapter, each static pressure port must be designed or located in such a manner that the correlation between air pressure in the static pressure system and true ambient atmospheric static pressure is not altered when the airplane encounters icing conditions. An antiicing means or an alternate source of static pressure may be used in showing compliance with this requirement. If the reading of the altimeter, when on the alternate static pressure system differs from the reading of the altimeter when on the primary static system by more than 50 feet, a correction card must be provided for the alternate static system.
(c) Except as provided in paragraph (d) of this section, if the static pressure system incorporates both a primary and an alternate static pressure source, the means for selecting one or the other source must be designed so that—
(1) When either source is selected, the other is blocked off; and
(2) Both sources cannot be blocked off simultaneously.
(d) For unpressurized airplanes, paragraph (c)(1) of this section does not apply if it can be demonstrated that the static pressure system calibration, when either static pressure source is selected, is not changed by the other static pressure source being open or blocked.
(e) Each static pressure system must be calibrated in flight to determine the system error. The system error, in indicated pressure altitude, at sea-level, with a standard atmosphere, excluding instrument calibration error, may not exceed ±30 feet per 100 knot speed for the appropriate configuration in the speed range between 1.3 VS0 with flaps extended, and 1.8 VS1 with flaps retracted. However, the error need not be less than 30 feet.
(f) [Reserved]
(g) For airplanes prohibited from flight in instrument meteorological or icing conditions, in accordance with §23.1559(b) of this part, paragraph (b)(3) of this section does not apply.
Also in my experience, just because you can't find any reference that says "it was designed for that scenario" in plain english doesn't mean that it isn't true. Often that kind of data is just not provided outside of the aircraft certification process.
Some food for thought, people always explained to me that the alt static pressure read inside of a cessna 172 was lower in flight because when you selected the alternate static pressure on the ground with the doors open it would read higher. Um, how would it read higher than the ambient on the ground with the doors open? That's physically impossible. What it boils down to is that it's just calibrated as specified in the POH, which is where you go, rather than trying to second guess all the dynamics involved with the instrument reading. In my personal opinion, the pressure is higher, because air vents and other sources "leak" air from in front of the cockpit into it, causing the air to "pile up" in the cockpit. The air in the cockpit is relatively still, while the air moving around the outside of the aircraft is moving fast. This generally means a pressure differential of high(inside) to low (outside), evidenced by just about every old convertible car you see on the road or someone who has used duct tape in place of a window. Maybe, just maybe, it reads higher on the ground with the doors open because in flight it IS actually higher pressure and it needs to read slightly higher to correct for the higher pressure (that will indicate lower altitude). That's just a guess though. We don't know for sure how they calibrate it and the error they publish and whatever it says in the POH is what should be followed.
I don't keep up on "systems" like I used to, as I'm required to be familiar with a much wider array of aircraft and systems, so unfortunately I can't go as deep as I used to. It's all still fun and interesting though!