When your dispatch release has two alternates listed, you might wonder why. Outside of ETOPS rules, there are two scenarios where you are legally required to have a second alternate. Last month, I covered “marginal” weather and the lack of standardized FAA definition of marginal weather. This month, I’ll cover exemptions allowed by the FAA. These are commonly misunderstood, so I will provide clarity from the dispatch perspective.
Scenario 2: FAA Exemption 3585 History
Launched in 1981, PEOPLExpress operated until 1987 as a low-frills air carrier in the post-1978 Airline Deregulation Act environment. A great article on its brief operation is found here: https://metroairportnews.com/peoplexpress-fly-smart/
Partway through its first year of operations, the upstart air carrier petitioned the FAA for an exemption from FAR 121.613 and 121.625 (at the time, these regulations were technically called Federal Aviation Regulations). The airline had repeated issues with weather reports and forecasts mentioning “occasionally,” “intermittently,” “briefly,” and “a chance of” weather conditions. PEOPLExpress was plagued by flight cancellations caused by National Weather Service forecasts with “chances” of poor weather and infrequent forecast updates. The carrier argued this was economically difficult and proposed adding a second alternate airport to the dispatch release whenever forecasts mentioned “occasionally,” “intermittently,” “briefly,” and “a chance of” conditions limiting their operations.
The FAA granted exemption 3585 to PEOPLExpress and “any other similarly situated Part 121 certificate holder” who wanted to have the exemption. Their original exemption can still be found here if you’d like to read more about PEOPLExpress’ request: https://drs.faa.gov/browse/excelExternalWindow/FAA00000000000000000000000EX3585.0001
Exemption 3585 / 17347 / 20108 Today
Over the years, the FAA continued extending 3585 until 2023, when it declared future versions would be renumbered as exemption 20108. Regional airlines were granted exemption 17347. Links to the relevant exemptions are at the end of this article.
Here is how the exemptions work. If the destination and/or first alternate have a TEMPO or PROB group in the TAF or remarks in the METAR showing weather conditions below the authorized weather minimums by no less than half the requirements, the flight can still be released under the exemption provided a second alternate is added to the release. The second alternate must fully meet the required weather conditions for ceiling and visibility.
To illustrate, our example flight is scheduled to KIAH (ETA 1510) and plans to use KHOU as the first alternate (ETA 1530).
TAF KIAH 271229Z 2712/2818 21006KT 1SM +SHRA BKN006
TEMPO 2714/2720 1/4SM +RA FG BKN002
FM272100 07006KT 2SM BR BKN004
FM280000 12005KT P6SM BKN250
We need to evaluate the weather for our destination, KIAH. KIAH has many ILS approaches with category I landing minimums of ½ sm. The TAF main body FM line at our arrival time of 1510 is fine with a visibility of 1 sm, but the TEMPO lowers it to ¼ sm which is below ILS landing minimums. Ordinarily we could not dispatch this flight due to the TEMPO. But, with the exemption, we CAN dispatch the flight to arrive at 1510 as long as we add a second alternate. This is because it is a TEMPO (conditional language) lowering visibility to ¼ sm and the minimum is ½ sm, which is half of what’s required. The ¼ sm is NOT in a FM line of the TAF.
Moving on to the alternate, let’s look at KHOU. Deriving alternate minimums for KHOU using operations specification C055 means we need a ceiling of 500´ AGL and visibility of 1¼ sm using the ILS 13R and ILS 04 using the 2 navaid rule.
TAF KHOU 271147Z 2712/2812 09004KT P6SM OVC005
PROB30 2712/2716 VRB15G25KT 5SM -RA BKN003
FM271700 15007KT P6SM VCSH SCT025 BKN035
Looking at the TAF for our proposed arrival time at KHOU (1530), we find the ceiling and visibility requirements are met except for a PROB30 forecasting a broken layer at 300´ = a ceiling. However, 300´ is over half of the 500´ ceiling required, and it is in a line with “conditional” language (PROB30). The minimum visibility of 1¼ sm is met as the lowest visibility at 1530 is 5 sm.
We now need to identify a second alternate. Let’s use KLCH with ETA of 1545. Using operations specification C055 again and the 1 navaid rule for the ILS 15, we derive alternate minimums requiring a ceiling of 600´ AGL and visibility of 1½ sm.
TAF KLCH 271128Z 2712/2812 14012KT 5SM -SHRA BR VCTS BKN025CB
PROB30 2712/2713 VRB20G35KT 1SM TSRA OVC015CB
FM271500 18010KT 6SM VCSH BR BKN030
FM272000 13008KT P6SM VCSH BKN040
Note that at 1545, both these requirements are met as the ceiling is forecasted at 3000´ and visibility 6 sm. Listing KLCH as a second alternate lets our flight depart on schedule. We also must carry enough fuel to fly to KLCH as it is farther away from KIAH than KHOU.
Here is a table summarizing the exemption in this specific example. Remember the ceiling and visibility with the exemptions can only be listed in “conditional” language (TEMPO or PROB) forecast groups. This exemption is only usable for domestic operations.
| Airport | Ceiling without the exemption | Visibility without the exemption | Ceiling with exemption | Visibility with exemption |
| KIAH (destination) | n/a | ½ sm | n/a | ¼ sm |
| KHOU (alternate 1) | 500´ | 1¼ sm | 250´ | ¾ sm* |
| KLCH (alternate 2) | 600´ | 1½ sm | 600´ | 1½ sm |
*Note: Half of 1¼ sm really is 5/8 sm, but TAFs don’t use eighths, so we round up to ¾ sm.
While PEOPLExpress ceased operations in 1987, their legacy lives on through these exemptions. The exemptions also require the aircraft dispatcher to be able to communicate with the flight crew and provide additional clarification on which alternate to use, since 14 CFR 121.639 requires enough fuel to be loaded to reach the most distant alternate (not to fly to BOTH alternates). As a reminder, always follow your company’s exemption and operations manual, and if you’re confused about fuel or alternate requirements or weather forecasts/reports, talk with your dispatcher!
To read the exemptions, here are links to the current ones (20108 and 17347):
https://drs.faa.gov/browse/excelExternalWindow/FAA0000000000000000000000EX20108.0001
https://drs.faa.gov/browse/excelExternalWindow/FAA000000000000000000000EX17347E.0001
I made a video about the topic for additional clarification. https://youtu.be/eIQdejMw9Zo or scan this code:
