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If the VVI/attitude indicators , FPM, airspeed indicator somehow failed same time and now you could not get cross control check, can use fuel flow counter(because of the decrease consumption rate) to help you with your attitude(climbing or descending) if your aircraft is in the clouds at the moment ?

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    $\begingroup$ Well, what would the fuel flow counter read in a spiral dive with the engine screaming? It may matter whether we are talking about fixed-pitch prop, constant-speed prop, or turbine-- so you might want to specify that in your question-- but the short answer has got to be "no, that the fuel flow counter is not a useful instrument for blind flying." For fixed-pitch prop, you will get some useful information from the tach and also just from the sound of the engine as another clue to rpms which is related to airspeed. $\endgroup$ – quiet flyer Jul 25 '20 at 14:03
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    $\begingroup$ PS I'm assuming you meant to say that the altimeter has failed too, otherwise it is just too easy as noted in answer below. $\endgroup$ – quiet flyer Jul 25 '20 at 14:06
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Without an Attitude Indicator, changes in altitude at a constant power setting gives you a fairly good indicator of pitch. If your Airspeed Indicator, Vertical Speed Indicator, and Attitude Indicator all failed at the same time, you would use your Directional Gyro and your Altimeter to stay straight and level. You would cross check that with your Magnetic Compass and your Turn Coordinator.

If your Altimeter fails you, you would keep wings level with your DG, TC, and Mag Comp while monitoring your Tachometer RPMs on a fixed prop or your Manifold Pressure on a constant-speed prop. This would give you a more immediate indication of pitch than your fuel flow counter. The DG, TC, and Mag Comp in most aircraft work off of separate systems from each other to avoid all three going inoperative at the same time. They are your instruments to indicate bank and turn.

The ASI, AI, VSI, and Altimeter are all pitot static instruments. And iced over pitot tube and static port can cause them to be inoperable. Using you alternate static port or breaking the glass face of your VSI will make your Altimeter work, making this scenario a non-issue.

Using a built-in GPS or an external AHRS like the Stratus will give you an unofficial altitude in a pinch.

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