421 driver here that's had the airplane for almost 3 years, I've got nearly 400 hours in her.
Here's the scenario:- Took the family from home base (Fort Worth Meacham) to see my parents and sister in California (specifically John Wayne, KSNA). Had a lovely time in California.
- Was thinking about returning on Sunday (1/14), but saw ice in the forecast over much of Texas, so the return trip was delayed to Monday, (1/15).
- The tailwind from California was pretty good on 1/15, but not enough to justify an attempt at going non-stop, so we stopped in New Mexico (KBRG) for fuel. Uneventful flight.
- Uneventful takeoff, climb, and cruise across New Mexico and Texas...no clouds the entire flight, as it was forecast. Outside temp at 15,000 was -5C. The forecast (and actual) temp in Fort Worth when we landed was -4C...cold day in North Texas. I was expecting a much colder en route temp at 15,000', so I assumed there was a temperature inversion (there was).
- ATC assigned me the Likes 3 arrival into the North Texas area (this is normal coming from the west), and Fort Worth Center gave a crossing restriction for the SLUGG intersection at 5,000 about 45 miles out (also basically normal).
- On the downhill, the computer was telling me that I needed 800 - 900 fpm to hit the crossing restriction. This is higher than my usual/preferred descent rate, but is not an unreasonable "slam dunk." My IAS rose to about 185 on the descent (yellow line starts at 200 and the air was smooth).
- As I got through about 12,000', the outside air temp started dropping quickly...it got down to -16C at some point (I don't recall the altitude). I probably don't recall the altitude because I became focused on my oil pressures, which were dropping during the descent. Normally during a descent, my oil pressures actually tick up a few points (from roughly 45 in cruise to maybe 47 or 48). But they slowly went through the 40s, and by the time I crossed SLUGG at 5,000', the right engine was in the upper 30s (still in the green arc, but not by much), the left engine was slightly below the green. The oil temperatures for both engines, which usually sit around 150 or 160, slowly climbed to nearly 200 (220 is top of the green). All 12 cylinder head temps remained cool as usual during the descent (low 300s).
- SLUGG is 28 nm from Meacham. Fort Worth Spinks (KFWS), which I am familiar with, is even closer. With the speed I had, it was going to be about another 8 - 10 minutes to get to Meacham, and at about SLUGG (where the oil pressures got to their lowest, and oil temps got to their highest), I started to contemplate where I was going to go if I went full dead-stick. The best options I could think of were: (1) Spinks, or (2) the relatively rural and undeveloped areas between SLUGG and Fort Worth. Between Spinks and Meacham is urban with few good options.
- Regional approach cleared me direct to Meacham and pilot's discretion on the descent. I started a very slow descent while watching the engines like a hawk...then the pressures started to rise to where they should and the temperatures started to drop to where they should. I noticed that the outside air temp started to rise, too. By the time I was closer to Meacham than Spinks, it was clear that the engines were happy and that I could make a normal visual approach to Runway 34. And so I did.
So my questions are:- Anyone ever experienced something like this before descending through a particularly cold layer of air in a temperature inversion? (Maybe not even in a 421, but bonus points if you did in a 421 and have special GTSIO-520 knowledge that applies here).
- Am I wrong altogether to assume this was related/connected/caused-by the layer of cold air I descended through?
- What mistakes did I make / what could I have done better?
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Last possibly relevant points to add to the equation: (1) Right or wrong, smart or stupid...I start every single flight with 13 quarts (the max) on each engine, including the flight in question.
(2) Without exception, I change the oil on both engines after 25 hours of flight (recurrent training instructor pounds this into his 421 customers, and I follow the advice). After landing in Fort Worth on the flight in question, I had 24.6 hours on the oil...so basically at the end of its shelf life.
And finally, a fair and general question might be: "Maybe try asking on Twin Cessna?" I will. But I've got a lot of respect for the BT community's database/wealth of knowledge so I wanted to start here.
Opinions, anecdotes, old 421 stories, and/or admonishments of my airmanship are all welcome, I'm here to learn and to be better.
Many thanks.
