Are You Prepared for Instrument Failure in IMC?

Photo: Wikimedia/Meggar

Autumn is in full swing, and the cooler nights tend to make morning fog a common occurrence in many places. While fog might not be a problem for you if you are IFR-rated and current, it’s nevertheless a good time to review your emergency procedures – like instrument failures and partial panel procedures.

A failure of any instrument in the cockpit of your airplane is difficult enough to deal with during a VFR flight, but the proper procedures after an instrument failure in IMC can mean the difference between life and death. While we tend to remain "current" by flying IFR flight plans and instrument approaches on a daily or weekly basis, unless you work for a company that requires it, you probably don’t practice instrument failures or partial panel procedures enough.

Are you ready for an instrument failure in instrument conditions? After training your eyes and brain to "trust your instruments," can you immediately recognize instrument errors and reverse that deep-rooted feeling that your instruments must be correct?

Identifying instrument failures seems like an easy enough task – after all, if an instrument is behaving erratically, there’s a good chance it’s malfunctioning - but it’s difficult for our brains to determine exactly what’s happening at first glance when an instrument fails, and sometimes the failure occurs slowly, such as the slow icing over of a pitot tube. And that’s only the first part of the emergency. The second part is responding correctly. While in the clouds without correct instrument indications, knowing which way is up can be puzzling to even the most experienced pilots. Here’s a quick review about how instruments react to common types of failures in many light aircraft.

***This is not a substitute for instruction. Please consult your aircraft’s POH for emergency procedures specific to your airplane! ***

Pitot-Static System Failure:
A problem with the static system will appear on the airspeed indicator, altimeter or vertical speed indicator (VSI), or a combination of the three.

  • Blocked Pitot Tube: A pitot tube blocked with insects is a common culprit of erroneous airspeed indications. This type of blockage might be noticed during takeoff, when the airspeed doesn’t increase as usual. With a total pitot tube blockage, the airspeed will read ‘0’. But the pitot tube can also be blocked during flight with ice or heavy rain, and as ice accumulates slowly over the pitot tube, the airspeed indicator will show a slow decrease in airspeed, maybe not even noticeable at first.

    Since the pitot tube is used just for the airspeed indicator, a blocked pitot tube will not affect the altimeter or VSI.

  • Blocked Static Port: A blocked static port isn’t too much of a problem if the aircraft is equipped with an alternate static source (many are). But without alternate air, a blocked static source will cause the airspeed indicator to act as a reverse altimeter, showing an increase in airspeed during a descent and a decrease in airspeed during a climb.

    With a blocked static port, the altimeter will freeze, showing the last altitude recorded before the blockage occurred, and the VSI will indicate ‘0’.

  • Pitot and Static Blockage: If both the pitot tube and static system are blocked, the airspeed indicator will act like an altimeter, showing an increase in airspeed when climbing and a decrease in airspeed while descending.

Gyroscopic System Failure:
There’s a reason the vacuum gage is checked during the engine run-up. It’s because two of the three commonly used gyroscopic instruments run on a vacuum-driven pump, and if these instruments fail, flying can be pretty dangerous.

The gyroscopic instruments typically include the turn coordinator, heading indicator and attitude indicator. The heading indicator and attitude indicator are vacuum-drive most of the time, so a vacuum failure or loss of suction will cause the attitude and heading indicators to ne unreliable.

Many commonly used turn coordinators are electrically driven (done for redundancy and as a backup to the vacuum system), and will fail along with an electrical failure.

A pitot-static or gyroscopic failure can be difficult to diagnose and confirm at first. The trick is to think about a failed instrument on a systemic level by determining which, if any, other instruments are also affected. If your airspeed seems off, check your other instruments. If they are also indicating erroneously, than you can bet there’s a pitot and static failure. If only the airspeed is incorrect you can rest assured the pitot tube alone is the culprit. By cross-checking often and making sure all your instruments agree with each other, you’ll be able to determine which are malfunctioning and take appropriate action. What’s the appropriate action? Covering up the inaccurate instruments and converting to a new insrtument scan that will keep you alive and allow you to land safely.

In any case, the quick and proper diagnosis of an instrument or system failure will turn an emergency into an inconvenience (although you should always declare an emergency when the situation warrants). A good pilot is always prepared, and preparation in this case comes with consistent practice, so be sure to brush up on your partial panel procedures often!