Night IFR to KDEN: Is the Alternate Real?
You are planning KICT-KDEN, departing 1930Z, ETE 2+10, in a PA-28RT-201T burning 11.0 gph with 48 gal usable aboard. KDEN TAF (1918/2024): 22014G22KT 4SM -RA BKN018 OVC040. KAPA TAF (1918/2024): 22015G25KT 6SM SCT025 BKN040. NOTAM: KAPA ILS RWY 35L unusable until 0300Z; localizer/VOR minimums 800-2 remain. Your ETA at KDEN is 2140Z. Plan on 0:10 taxi, 0:20 for the approach at KDEN, 0:25 KDEN-KAPA, and a 45-minute reserve at cruise fuel flow. The only usable approach at KDEN tonight is an ILS to 200-1/2. The airplane has a current-database IFR GPS, and both airports have approved approaches.
1. Is a destination alternate required for this flight, and does KAPA satisfy the requirement? Show the rule you applied rather than the conclusion.
Read the worked answer
Yes - required, and KAPA works.
§91.169(b) requires an alternate unless the destination has a standard instrument approach and the forecast, from 1 hour before to 1 hour after your ETA, is at or above 2,000 ft ceiling and 3 SM. ETA 2140Z, so read the TAF from 2040Z to 2240Z.
- KDEN: BKN018 = 1,800 ft, below the 2,000-ft test. Visibility (4 SM) passes, but one failing element is enough. Unknown to many pilots, this rule applies even when the destination has a perfectly good ILS to 200-1/2 - the 1-2-3 rule is regulatory and has nothing to do with whether you could actually land.
- Consequence: an alternate must be named on the filed IFR flight plan.
KAPA test - compare the forecast at the ETA at the alternate (about 2205Z, window 2105Z-2305Z) against the standard alternate minimums: 600-2 if a usable precision approach exists, 800-2 for non-precision. Because the ILS is NOTAM'd out, the 600-2 credit is gone and you must use 800-2. Forecast 2,500 scattered, 4,000 broken, 6 SM is far above that.
Confirm the A/FD alternate-minimums section carries no 'NA' note for KAPA. On these numbers, KAPA is a valid alternate.
2. Compute the minimum fuel required at brake release and compare it with the 48 gal aboard. Is the flight legal, and how much real margin do you have?
Read the worked answer
42.2 gal required; 48.0 aboard; about 32 minutes of margin. Legal, thin.
§91.167 requires fuel to the destination, an approach, a missed approach, the alternate, and 45 minutes at normal cruise. Sum the segments:
| Segment | Time |
|---|---|
| Taxi/runup | 0:10 |
| En route | 2:10 |
| Approach at KDEN | 0:20 |
| KDEN-KAPA | 0:25 |
| Reserve | 0:45 |
| Total | 3:50 |
3.833 h x 11.0 gph = 42.2 gal.
48.0 - 42.2 = 5.8 gal = ~32 minutes at cruise flow.
The flight is legal, but the cushion assumes exactly one approach, no hold, and the planned alternate. One 15-minute hold at KDEN, a second approach, or a stronger-than-forecast headwind to KAPA eats half of it. The 45-minute reserve is a regulatory floor, not a target - for a night IFR flight into an 1,800-ft ceiling I would either top off (5-6 gal is cheap) or choose an alternate closer than 25 minutes.
3. At 2100Z, while you are still on the ground, the KAPA TAF is amended to 22018G30KT 3SM -RA BKN008. What changes, and what do you do with the information?
Read the worked answer
It is still legal - and that is exactly the problem.
With the ILS out, KAPA's alternate standard is 800-2. The amendment gives a ceiling of exactly 800 ft and 3 SM, both at or above the minimums, so KAPA technically still qualifies (the test is 'at or above,' not 'above'; gusts to 30 kt do not enter the test at all).
But you now have an alternate sitting precisely on its floor, at night, in gusty rain, with no second alternate. Every factor that erodes a forecast - a slightly lower ceiling, a heavier shower - removes your legal alternate in flight.
Options, in order of preference:
- Delay until the amendment's trend is clear or a second viable alternate exists.
- Add fuel and file a second alternate (a return airport behind you) so the loss of KAPA does not leave you with nothing.
- If already airborne when this happens, decide early - pick a new destination while you still have fuel to reach it, tell ATC, and expect to lean on §91.3 if the weather goes below minimums before you land.