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“Descend and maintain” is such a staple of IFR communications, it might as well be a single word. Yet there are times when that’s the last thing you want to do. Maybe those clouds are bumpy, and you have the family on board. Maybe they’re icy, and you need to minimize your exposure. Maybe you just like the tailwinds where you are and want to keep them for as long as possible before you absolutely must descend to land.
If ATC issues a descent you’d like to delay, reply with: “Request descent at pilot’s discretion.”
It Never Hurts To Ask
Pilot’s discretion on descents (or climbs) means the pilot essentially owns a block of altitude from the current one down to the new altitude. The pilot can change altitude when he or she sees fit; however, that pilot can’t return to an altitude once the pilot vacates it. So that “block” gets smaller and smaller until the assigned altitude is reached.
This happens more often over sparely populated areas, where ATC might volunteer the clearance, “…descend at pilot’s discretion, maintain 3,000.” However, there’s nothing stopping you from asking for that freedom whenever you want it. It’s a regulatory truth that pilots must comply with ATC clearances, but you can still buck for a better deal before you accept it.
Keep your request as clear as possible because you’re asking for something out of the ordinary. “Approach, Two Six Delta, we’d like to remain at 6,000 until we can get an uninterrupted descent to 3,000 to minimize our time in the turbulent/icy/scary/icky [circle one] clouds. Request pilot’s discretion to 3,000.”
You might be told that can’t happen for another five miles. Fine. Start planning how fast you’ll need to come down vertically and how much ground you’ll cover horizontally. You might need to slow your groundspeed right now. The last thing you need is a situation where you get your request for a rapid descent only to discover you’re too fast to drop gear and flaps and get established on a stable glideslope—or unprepared for the approach entirely.
You may get a crossing restriction, such as “…descend at pilot’s discretion to 3,000. Cross FIXIE at 3,000.” This puts the ball in your court. What will it take to stay high as long as possible but still cross FIXIE at 3,000? There are two ways to look at that, depending on how low you want to go.
Suppose 3,000 feet is below the problem layer, or you’re staying high just to keep a nice tailwind as long as practical. In this case, maybe you want 1,000 feet per minute downhill and would like enough time to slow down a bit before FIXIE. Give yourself three minutes for the descent, plus maybe two minutes to slow down. When your GPS shows a bit more than five minutes from FIXIE, start down. Remember, descent is at pilot’s discretion. You can notify ATC you’re starting down, but it’s not required unless ATC added a “…report leaving [altitude]…” Adjust power to roughly hold your groundspeed with the changing winds, if you want, and level off at 3,000.
If your GPS has a vertical speed required field, or vertical profile, that works great as well. But the math here is pretty simple either way, and the required result has room for error.
It’s more challenging if 3,000 feet is smack in the middle of the layer you want to hurry through. Now you want to reach 3,000 feet in a rapid descent and cross FIXIE at the same time. This situation comes up more often in the approach environment where you’re in control of your own stepdowns, but it still takes planning.
Suppose you’re flying the RNAV (GPS)-D to Rangeley, Maine (8B0), and you’re approaching SHINY. You’re at 7,000 above a layer that starts at 6,500 with bases at 3,000, and you’re concerned about icing or turbulence in those clouds. You know from experience that ATC normally descends you to 6,000 to cross SHINY, which is at the top of the clouds and exactly where you don’t want to linger.
You request and receive pilot’s discretion for descent to 3,800, cross the RQM NDB at or above 3,800, cleared for the RNAV (GPS)-D approach. You have essentially 3,000 feet to lose before the NDB and about 1,500 feet to lose after it. You want to descend quickly but controlled, so you plan for 1,000 fpm. Or maybe 1,500 if you really want to keep it short. You don’t want a lot of forward speed because you want time to level off and stabilize before the missed approach point at ROOTS.
Slow down and watch your groundspeed while holding 7,000 feet, and when you’re three minutes from the NDB, pitch down and descend, adjusting as needed to keep your groundspeed relatively constant. (There’s a separate discussion here about descending through potential icing with flaps and gear out for drag or in, to prevent ice attachment, but that’s for another time.) You should cross over the NDB just a shade before reaching 3,800 feet, so you can keep going down. As you approach the MDA, you’ll (hopefully) break out and can transition to level flight while looking for an airport.
With any request from ATC, you should be prepared to negotiate: “We can accept vectors off course if that helps. Thanks.” Remember that altitude changes have three primary purposes in the controller’s mind: To keep aircraft separated, to give you a reasonable transition onto the approach at your destination, and to meet any local airspace procedures as to who controls which aircraft in which blocks of sky. You and the controller may need to get creative to meet everyone’s needs. Freedom to not descend may cost you some time waiting for the opportunity or on a longer route. You may have to give in order to get. While it’s less common, pilot’s discretion can work with climbs as well.
Of course, if the situation warrants it, you can always use the E-word and get any altitude you need, but we’re trying to keep this low-key. Remember, the family is in the back.
There are a couple other tools that we often forget about that can be helpful here. Let’s say you’re really not sure about those clouds 500 feet below you. Will they have ice in them or not? Asking for pilot’s discretion doesn’t let you go back up once you vacate an altitude. A block altitude might be a better tool.
If you were at 6,000, you could request “…6,000 block 4,000 for about 10 miles…” Once approved, you own all altitudes from 6,000 down to 4,000 and back up. Now you could dip your toe into the troubled altitude and see if it’s likely to be an issue. If not, great; you can convert your block altitude back into a solid IFR altitude and continue. If there is an issue, you can retreat back to 6,000 and plan your next move.
Another tool is a cruise clearance. You won’t get one coming into White Plains, New York, but you can get them at low-traffic areas or hours, like Rangeley, Maine. The beauty of the cruise clearance is it’s pilot’s discretion all the way to the pavement. If you request “…cruise 6,000,” and it’s approved, you can stay at 6,000 as long as you want, even as you get established on a published approach. You can go as low as the MEA or published minimum for any segment of the approach—and that’s any approach at the airport you want. You could even cruise to a point in the clear on a published route and then request a contact approach to stay out of the clouds to make it to the airport visually. Cruise clearances are unusual enough the controller might have to pull out their 7110.65 ATC bible and look it up, but that’s not your problem.
If you want to play with altitude and can do it all in clear air above a cloud deck, you can ask for VFR-on-top. VFR-on-top is used in lieu of a fixed IFR altitude. So you’ll be told to “maintain VFR-on-top” rather than “maintain 6,000.” You now fly the appropriate VFR altitude for your direction of flight as you wish, but you must maintain VFR cloud clearances even though you’re still on an IFR flight plan.
Sometimes it’s both altitude and route you want to experiment with. When I lived in Seattle, we had a local procedure called a shuttle climb. The issue was that climbing eastbound over the rising terrain toward the mountains in winter might turn you into a flying Slurpee. The solution was requesting a “shuttle climb over Puget Sound.” This was a series of vectors, first north, then south, then back north again, and so on, until you broke out on top or felt comfortable turning east. It was common to find no ice at all or just a trace of rime, climbing over the Sound even when there were PIREPs of nasty ice by the mountains. Even if you did find some ice, you had low, warmer terrain below to turn tail back home.
That’s the beauty of knowing what to ask for. Sometimes ATC couldn’t care less what altitude you fly, while for you it makes all the difference.
You can read more on this subject, or other instrument flying topics, in Jeff’s IFR Focus blog.