Using FPA for Slowdowns
Introduction
You’re coming down the STAR in your E175 when approach gives you “Airline 123, slow 210 then descend 8000,” or “Slow 210, high on the arrival approved.” The obvious choice is to go manual speeds 210, 8000, and then hit green FLCH. While that works, it’s not the most elegant solution if ATC wants you slowing down now.
The problems with FLCH
FLCH in the E175 has two behaviors that make it a suboptimal way to slow the aircraft on autopilot:
- It doesn’t pitch up with an awful lot of urgency, so you spend a lot of time at your faster speed and then ATC gives you an even slower assignment, delay vector, or something else.
- FLCH likes to keep some descent doing, usually 500 FPM. This eats into your ability to slow down when it’s more urgent to get your speed dialed before altitude.
The solution
If ATC wants you to slow first, then go down, you can pull the power out and pitch up to a level flight path to bleed speed, then hit FLCH to keep the descent going. A couple techniques to do that:
These techniques all have their tradeoffs. If your PM has not seen these used yet, brief your plan ahead of time so that they don’t get confused about your intentions.
If you’re close to your desired profile anyway and a brief level-off will put you too high (e.g. slam-dunk to 28R at SFO), go ahead just leave it in PTH mode and pull the boards. If you have all the time in the world and need to save fuel (e.g. long conga line at SEA), this is a great situation to use it.
- Dial in speed 210 and 8000, then hit FPA and twist for 0.0. The airplane will chase the pitch target more aggressively than with FLCH, thus helping you slow now. When your speed comes down through 220 (or 10ish KIAS above whichever bug speed), hit FLCH and let it descend to 8000 on speed.
- Dial in speed 210 and 8000, then hit green FLCH and hold TCS while pulling the nose up to an FPV of 0.0. When it gets close to target, let go of TCS and the plane will chase the descent at the target speed.
- Turn off the automation and pitch it up like you’re flying a Cessna. Trim for 210, then nudge it about 3 degrees down to hold speed while descending.
These techniques all have their tradeoffs. If your PM has not seen these used yet, brief your plan ahead of time so that they don’t get confused about your intentions.
If you’re close to your desired profile anyway and a brief level-off will put you too high (e.g. slam-dunk to 28R at SFO), go ahead just leave it in PTH mode and pull the boards. If you have all the time in the world and need to save fuel (e.g. long conga line at SEA), this is a great situation to use it.
Fuel Savings
Converting speed to altitude helps keep you at idle thrust longer and thus saves fuel. Pulling the boards, by contrast, wastes the kinetic energy by turning it into drag. If we use the same calculations from the slowdown article and assume a 72,000 lb airplane at 250 KIAS and 7,000 feet slowing to 210 KIAS, we have (rounded numbers) an initial energy of 395.7 MJ and a final energy state of 279.2 MJ for an energy delta of 116.5 MJ. Jet-A has a specific energy of 43 MJ/kg. If we assume a partial throttle propulsive efficiency of .7 and a Carnot efficiency of .3, we end up with:
Thus, pulling the boards wastes the equivalent energy of 20ish lbs of fuel if we need to spool up earlier on the next level-off. At 6.8 lbs/gal and $3/gal, that works out to about $10. On a fleet of 250 doing it 2 times a day, you would save $1.9M annually.
Conclusion
FLCH is not always your best friend. Sometimes FPA or TCS can get you on speed earlier, then you can transition back to FLCH for the rest of your descent. Using these techniques will often help you conserve total energy, thus reducing the need to pull boards or throttle up later on. As always, experiment in lower-workload situations and share your mental model with your PM.