A student and instructor received a "low altitude alert" from ATC after forgetting to reset their altimeter before an instrument approach. After a long flight, making this simple mistake could leave you hundreds of feet too high or too low on final approach.
It's An Easy Mistake To Make
The following NASA ASRS report was published in February by a CFII giving instruction to an instrument student while flying a Cessna 172...
I was acting as CFII with Instrument ASEL student on training flight, executing the LOC 23L approach at LCK, Glideslope Out. During a previous approach, we received an altimeter setting of 30.21. After the first approach, we re-tuned the ATIS frequency and received new ATIS information at LCK. We confirmed this with Columbus Approach. The student did not reset the altimeter to the LCK setting of 30.16 and I failed to cross-check the student's procedures.
We descended to the MDA at an indicated altitude of 1,220' MSL after passing the FAF, at which point LCK Tower gave a low altitude alert and confirmed that our altimeter setting should've been 30.16. We immediately executed the missed approach procedure. At the time of executing the MAP, we had ground visual contact but did not have the runway in sight.
Are You Flying Into A Non-Towered Airport?
These pilots were fortunate that they were flying into a towered airport with controllers monitoring their altitude along the approach. ATC was able to issue them a low altitude warning, prompting their immediate missed approach. In this case, the altimeter change was relatively small, but enough to prompt the low altitude warning.
The majority of airports around the country do not altitude alerts available as a safety net for pilots. For instance, if you're flying into a non-towered airport on an instrument approach, Center or approach control will switch you over to the airport's CTAF frequency well before you reach DA or MDA. No one will be on-frequency to warn you that you've descended below the minimums for that approach. This could put you dangerously close to the ground, especially during precision approaches with low visibility or ceilings.
In a non-radar environment, ATC will not be able to provide you with the same altitude alerting capabilities that you'll find along instrument approaches into larger airports. During long cross country flights you're more likely to see dramatic altimeter setting changes, which could leave you with an entirely inaccurate indicated altitude reading.
The Solution? Add A Mental "Trigger" To Your Approach Briefings
Updating your weather information before beginning an approach is critical. In low conditions, having an accurate altimeter setting will ensure you can safely fly to the lowest altitude along the final stages of an instrument approach. As you reach minimums, you're usually quite close to obstacles below you, so just a few clicks of the altimeter setting can make a huge difference on your height above terrain.
As you brief your approach, use the "frequencies" section of the plate as a mental trigger to ensure you have updated weather information. Try mentioning your updated altimeter setting as you read over the airport's weather frequency. If you begin briefing the approach and realize you haven't updated the weather, pause the briefing and tune the listed AWOS/ATIS frequency.
These pilots did a great job realizing their mistake and immediately executing a missed approach. If you ever find yourself in the same situation, execute the missed approach immediate and set yourself up again.
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