ForeFlight Flight Planning Errors

Discussion in 'Flight Following' started by Eric Reyes, Mar 5, 2018.

Tags:
  1. Eric Reyes

    Eric Reyes Filing Flight Plan

    Joined:
    Mar 5, 2018
    Messages:
    7
    Display Name:

    Display name:
    Eric R
    I have found a number of errors in the ForeFlight Flight planning engine.

    I planned a flight in a Cessna 172S traveling 55 miles at 4,500 feet at 2,400 rpms with a cruise true air speed of 108 knots. I exported this flight from the Maps page to the Flights page and then viewed the Navlog on the Flights page. The NavLog automatically calculates the top of descent (TOD) waypoint, but the true air speed show on the NavLog from top of descent (TOD) to my destination is only 53 knots. That crazy slow!

    The stall speed clean config in the 172 is 48 knots. No way would anyone descend from TOD at only 53 knots TAS. I selected the standard performance profile in ForeFlight for the Cessna 172 and it doesn’t allow you to edit the descent speed or RPM’a and the only information it provides on the aircraft performance profile page for descent is “500 feet per minute”. There is no way to edit the descent rate or speed in a standard performance profile.

    Any advice? Am I crazy or is this 53 knot true air speed from top of descent way too slow? I even tried a few different airplanes with the stock performance profiles and they were all way too slow for true air speed from top of descent. For example the Cirrus SR22T had a true air speed in the NavLog from top of descent of 74 to 87 knots. That way too slow for decent speed in a SR22T. Same thing for the Piper M350 Mirage.

    The other anomaly that I found in ForeFlight is the course on the Map page NavLog being 2 to 4 degrees different from the bearing on the Flights page NavLog. On the Maps page I chose a departure date 1 week in advance so that the Maps Page NavLog will show course instead of heading. I then exported the flight to the Flights page and viewed the Flight page Navlog. On the Flights page NavLog there is a column titled BRG (bearing). The bearing between waypoints show here on the Flights page NavLog are 2 to 4 degrees different from the courses between waypoints shown in the Maps Page Navlog.

    Correct me if I’m wrong but shouldn’t course and bearing be the exact same when in the pre-Flight planning stage and the assumption is that you stay on course so your bearing should always be equal to the course?

    What am I missing here?
     
  2. denverpilot

    denverpilot Taxi to Parking

    Joined:
    Nov 8, 2009
    Messages:
    48,653
    Location:
    Denver, CO
    Display Name:

    Display name:
    DenverPilot
    The Navlog calculates the TOD using the performance profile under the More tab, Aircraft, for the specific aircraft you define or the default aircraft.

    I suspect if your airspeeds for climb and descent are way out of whack, the performance numbers aren’t set right for the aircraft from POH numbers.

    An example might be, I just stuck a local club 172 into it and let it download the default performance numbers for that model, and it didn’t fill all of them in. The default performance database is not necessarily correct. You have to check it for your specific aircraft.

    I also poked a quick flight into it with my personal aircraft and the checked performance numbers and it showed correct TAS numbers (as requested for climb and descent in the profile) for the flight.

    As far as the heading and course, ForeFlight does winds aloft corrections. The old way to get a no-wind flight plan out of it was to put in a ridiculously high altitude so it couldn’t get winds aloft information, but you’re trying to check the vertical profile too, so that won’t work. I’d have to look in the manual to see if there’s a way to get a no-wind flight plan out of it with correct date/time and also correct altitudes.

    Look at the winds aloft forecast for your chosen altitudes and see if that explains your course vs heading difference.

    Also make sure you’re looking at the TAS column and not the GS column for the speeds. It’ll calculate your expected groundspeed with winds also.
     
  3. Eric Reyes

    Eric Reyes Filing Flight Plan

    Joined:
    Mar 5, 2018
    Messages:
    7
    Display Name:

    Display name:
    Eric R
    Thanks. I’m definitely looking at true air speed (TAS) in the NavLog. On one flight with a headwind, the NavLog had me traveling at 54 KTAS from top of descent to destination and my ground speed was only around 45 knots due to a head wind.

    You are correct that this slow true airspeed are coming from the Performance Profile for that airplane, but ForeFlight now provides performance profiles by airplane type and model. So when I set up my airplane (Cessna 172S) in ForeFlight, the app automatically imported the performance profile that I assume they got from the POH. The problem is that when you use these imported performance profiles ForeFlight does not allow you to edit/adjust the climb and descent performance. There is a % slider to adjust cruise performance, but for climb it is locked at “full power” and for descent it’s locked at “500 fpm” and neither actually tells you what the climb and descent numbers are behind the curtain.

    However when I created a Flight plan and viewed the NavLog on the Flights page, that where I see a super slow true air speed from top of descent on. I even set up profiles for a number of other planes that ForeFlight has performance profiles for in their system. Every plane I tried had true airspeed from top of decent at about half of the normal cruise speed. A Cirrus that is cruising at 150 TAS has a decent speed of around 70 KTAS.

    It’s looks ForeFlight really screwed up on the descent speeds for all of the Performance Profiles that they have available in the app for a number of different planes.

    So until ForeFlight fixed this, I can just ignore the NavLog figures from top of descent on or I can build my own customer performance profiles using my airplanes POH, but that very time consuming. Hopefully they will get this fixed. I can’t believe I’m the first person to noticed a 54 knot descent speed for a Cessna 172S.

    These true air speeds from top of descent are even a little slower than appropriate descent speeds while on short final. And depending on the cruise altitude, you could still be 10 to 30 miles from the destination when you reach top of descent. That means that the Navlog has you traveling at below final approach speeds for 10 to 30 miles which throws off all of the other NavLog calculations from top of decent onward including ETE, ETA, fuel, ground speed, etc.
     
  4. Eric Reyes

    Eric Reyes Filing Flight Plan

    Joined:
    Mar 5, 2018
    Messages:
    7
    Display Name:

    Display name:
    Eric R
    Ok. ForeFlight sent me a reply to my support ticket. They said that that use the POH descent speeds for a normal descent for their performance profiles, but then they add some extra time for flying the pattern and landing.

    So a 90 KTAS descent for 10 miles from TOD to entering the pattern plus 5 minutes extra for flying the pattern and landing, might show on the NavLog as only 57 KTAS from top of descent to landing.

    The confusing thing is they don’t show you these calculations anywhere so all you see is a slower than normal descent speed in order to account for the extra time for pattern and landing.
     
  5. denverpilot

    denverpilot Taxi to Parking

    Joined:
    Nov 8, 2009
    Messages:
    48,653
    Location:
    Denver, CO
    Display Name:

    Display name:
    DenverPilot
    That seems odd because no mayyet what I did I always got my “requested” descent airspeed on the log out of it as long as it was in the profile.

    And I’ve seen errors of both wrong speeds and missing speeds for some aircraft models in their database. On your imported default profile for the aircraft are all fields filled in and accurate to your POH? The 172 tail number I tried was significantly incomplete after importing their profile. You definitely have to check them.
     
  6. NoHeat

    NoHeat En-Route

    Joined:
    Jul 27, 2009
    Messages:
    3,582
    Location:
    Iowa City, IA
    Display Name:

    Display name:
    17
    I didn’t know Foreflight had such a feature in its Navlog. So I looked in the Pilot's Guide, and it mentions that there are two kinds of Navlog. I was familiar only with the one on the map page, which has very little detail. It turns out there’s another Navlog, much more detailed, on the flights page, just as you said, in a tab next to the Briefing tab. I had never noticed it!

    The descent point it proposes for my recent flight is just right, by the way. No error for me.
     
  7. BrianNC

    BrianNC Pattern Altitude

    Joined:
    Feb 2, 2009
    Messages:
    1,910
    Location:
    Atlanta
    Display Name:

    Display name:
    BrianATL
    Yep. Once you create the flight plan on the map page, then you hit the upload button in the lower right corner of the Edit/Navlog drop-down and upload it to Flights, Logbook, email it, etc.
     
  8. Eric Reyes

    Eric Reyes Filing Flight Plan

    Joined:
    Mar 5, 2018
    Messages:
    7
    Display Name:

    Display name:
    Eric R
    So the airspeeds for climb and cruise in the Nav log are always spot on. The problem is just the last led from top of descent into the arrival airport.

    I double checked the performance profile for the aircraft which is a 172 and everything appears correct. Because I went with the ForeFlight profile for the 172S, there isn’t any customization that you can do tp the climb and descent performance. There are 5 profiled to chose from for the 172 when planning your flight including: max lean, long range cruise, 2300 rpm lean, 2400 rpm lean, and 2500 rpm lean. But for all 5 profiles the climb just says “full throttle and the descent says “min RPM 500 FPM”.

    I have compared 4 of my pre-flight NavLog to actually the stats on the actual cross country flights and for the most part they have been spot on for distances, times, speeds, fuel, etc. Other than the fact that the TAS from top of descent is always about 58 to 64 KTAS in the NavLog but in reality I’m usually traveling around 85 to 95 KTAS while descending at 500 to 750 FPM at 1600 to 1900 RPM’s.

    I spoke to a rep from ForeFlight yesterday and she explained again that the speed they show in the NavLog from top of descent to destination in artificially made much slower than reality in order to account for the extra time or enter the pattern, fly the pattern, and land.

    So let’s says that I am flying 15 miles from top of descent to destination at an average of 90 KTAS and just for easy math 90 knots ground speed. That final leg time would be 10 minutes. However in reality it’s probably going to you 15 minutes to actual landing. So ForeFlight adds that extra time and then recalculates the ground speed and true air speed for that last leg. So in the NavLog would show 60 KTAS and 60 knots ground speed (assuming no wind) for that last leg from top of descent to destination.

    This is what original threw me for a loop until ForeFlight explained what’s happening behind the scenes.
     
  9. denverpilot

    denverpilot Taxi to Parking

    Joined:
    Nov 8, 2009
    Messages:
    48,653
    Location:
    Denver, CO
    Display Name:

    Display name:
    DenverPilot
    Definitely strange. I don’t like their methodology if what the rep is saying is true.
     
  10. Hank S

    Hank S En-Route

    Joined:
    Jul 3, 2013
    Messages:
    4,555
    Location:
    Eclectic, AL
    Display Name:

    Display name:
    Hank
    So what would happen on my typical flights where I cruise 7500-10,000 msl at 145-150 KTAS, and start down ~50 nm from my destination at 500 fpm without reducing power? Sometimes I see groundspeeds ~200 knots, while indicating 170 mph = 148 KIAS.