Using FPR to Catch Mountain Wave
Intro
Getting caught in mountain wave at altitude is no fun. You’ve got very little extra thrust, you’re trying to hold altitude in RVSM airspace, and you have reduced buffers between overspeed and underspeed. Fortunately, there’s a little-used tool in the E175 that can help us catch it more quickly and prompt us to take action: the humble flight path reference line.
The Problem
The E175 autopilot will prioritize holding altitude in SPDt modes, especially in RVSM airspace. If you start to encounter mountain waves that push your airplane up or down, the autopilot will quickly adjust pitch to maintain the assigned altitude. In an updraft, it will pitch down and accelerate, while a downdraft will make it pitch up and decelerate. The autothrottles (ATs), ever late to the party, will then gradually figure out that the speed is outside the bug and start to chase the target. If you started with the updraft, the ATs will keep pulling back to chase the bug speed, unaware that you’re about to crest the wave. When that happens, you’re in a downdraft, and your engines are down at 70% N1. Now the plane pitches up to hold altitude and your speed is trending slow. Your thrust is low, so now the ATs need to spool back up past the baseline and pile on more thrust (which you don’t have much of) to get back to target. Catching this cycle early will help you manage your thrust more proactively to keep you in the middle of your energy box.
Using FPR
If you come up on a region of suspected mountain wave (e.g. anywhere over or in the lee of the Rockies), pick a speed that gives you adequate margins on both sides and hit the FPR key (not FPA) on the guidance panel to draw the dashed blue line on your PFD. Next, twist your FPA knob to put the line through the middle of the black crossbars. Then note the angle (2.8 in our drawing) and your N1 (let’s say 86% for our example).
Getting a baseline FPR, speed, and N1 setting will help you spot deviations faster.
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The plane will adjust pitch to hold altitude, moving the crossbars off your reference line.
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The FPR line is a bit narrower than the crossbars, which makes it easy to spot if it’s off-center. A small deviation of .1 degree will show up quickly as the crossbars wander off the FPR line.
Riding the waves
If you see evidence of an updraft (crossbars below the line), ghost the throttles and expect them to start easing as the speed climbs. Depending on the speed gain, you might want to jockey the throttles a bit to keep them from coming back too aggressively. Maybe let them ease to 80-82% N1 to arrest the trend, then let the speed come back toward the bug gradually. If your speed trend vector starts coming back quickly, check your crossbar/FPR to see if you’re in the downdraft (crossbars over FPR line). If you are, get your N1s back to at least 86% (or whatever your baseline value was) and don’t let the speed drop too far below the bug. Keep in mind that the ATs are reactive and have no idea that you’re surfing through the trough. They would rather chase the bug while you would rather have a few extra knots to trade back in the updraft.
Another technique some pilots use is to let the ATs stabilize before the wave onset, then disconnect them and allow the speed to wander a bit as they go through the crests and troughs. If the amplitude of the waves is in the 10-20 second range, this method avoids allowing the ATs to magnify the energy problem by chasing the airspeed out of phase as discussed above (updraft -> speed gain -> AT chases speed -> downdraft at low thrust setting -> rapid speed decay). You’ll oscillate around your target speed, but your average speed should be about what was before if you keep your thrust setting. If the oscillations get outside a pre-defined margin (e.g. 10 knots), adjust the throttles on the peaks and troughs (as indicated by passing your FPR line) to dampen the excursions. Start with a small amount like 3-5% N1 and add more if you get near the edge of your speed window.
Sometimes the downdrafts will be strong enough that you won’t have enough extra thrust or speed to trade while holding altitude. In that case, it’s your job to tell ATC that you need to trade altitude back for speed and descend.
Another technique some pilots use is to let the ATs stabilize before the wave onset, then disconnect them and allow the speed to wander a bit as they go through the crests and troughs. If the amplitude of the waves is in the 10-20 second range, this method avoids allowing the ATs to magnify the energy problem by chasing the airspeed out of phase as discussed above (updraft -> speed gain -> AT chases speed -> downdraft at low thrust setting -> rapid speed decay). You’ll oscillate around your target speed, but your average speed should be about what was before if you keep your thrust setting. If the oscillations get outside a pre-defined margin (e.g. 10 knots), adjust the throttles on the peaks and troughs (as indicated by passing your FPR line) to dampen the excursions. Start with a small amount like 3-5% N1 and add more if you get near the edge of your speed window.
Sometimes the downdrafts will be strong enough that you won’t have enough extra thrust or speed to trade while holding altitude. In that case, it’s your job to tell ATC that you need to trade altitude back for speed and descend.
Conclusions
Mountain wave often starts smoothly and the airplane does a good job of adjusting pitch to hold altitude. This can make it harder for us to observe it as pilots if we wait for speed or altitude to wander. Using FPR will give us an earlier heads up about the pitch change so that we can manage our energy state more proactively.