Friday, April 24, 2020

Percent Power

We often see references to 75%, 65% or some other percent of max power setting.  It's easy if a pertinent chart/reference has the correlated RPM setting, but many do not.  When there isn't an RPM setting, it is completely frustrating.  For example, if your max RP is 2700 RPM, 75% power is NOT 2025 RPM.  It's actually 75% HP - which makes sense - but how do you calculate that?  It would seem if 2700 was 100% HP, 2025 would give you 75% HP...but it does not.  

In studying engines and this problem, I've come up with a possible equation to calculate your percentage RPM settings.  I underlined "possible" because I haven't correlated it to every engine.  It goes like this - and it's pretty simple:
  • Subtract the percentage from 100, multiply by 10, subtract from MAX
  • Example:  75% of 2700 and 65% of 2700
    • 100-75 = 25 x 10 = 250.  2700-250 = 2450 RPM (75% RPM)
    • 100-65 = 35 x 10 = 350.  2700-350 = 2350 RPM (65% RPM)
I hope this helps.  This question has bothered me for quite some time and it's a question students ask.

- FIG -

Thursday, April 23, 2020

Instruments : Approaches

In general
  • Approach Title:
    • If the procedure ends in "A, B, C..." there are multiple circling approaches
    • If the procedure ends in "Z, Y, X..." there are multiple ST-IN approaches
  • Briefing:
  • Chart review at
  • Charted Visual Approach (boldmethod)
  • ILS
    • If your right wing is on the shaded part of the feather, then steering is "right" (correct). If it's your left wing, steering is "wrong" or back course.
  • LP approach
  • LPV and LNAV/RNAV, the differences:
    • They’re both GPS approaches with vertical guidance, but similarities end there.
    • LPV
      • Use WAAS/GPS, but NOT precision.  They are APV (approach with vertical guidance) - it was less admin and cost to call them APVs.  BARO-aided GPS doesn’t suffice.
      • They get more sensitive as you get closer and are about 700’ at the threshold (like an ILS), but they essentially turn linear after the threshold
      • Can’t use precision alternate minimums.  You need WX that meets LNAV, circling or LNAV/VNAV DA
      • Actually, first GPS approaches with vert guidance – designed for BARO-aided GPS
      • Difference?  Don't have increasing angular guidance as you approach the runway.  They decrease to 0.3 nm sensitivity when w/in 2nm of FAF – all the way to MAP.
      • The lowest they can go is 250’ above touchdown, but due to obstacles it’s often higher
    • LNAV +V
      • Only shows on your GPS (if able), not on plates.  And the vertical glide path is advisory only…you still need to fly step-down altitudes and MDA
  • RNAV (GPS) Approaches:
    • Great explanations by:  FAA and boldmethod
    • Remember, even with vertical guidance, and while using a DA (decision altitude), these are NOT considered precision approaches.  They are APVs (APproaches with Vertical guidance).  If you remember "guidance" and "glideslope", it will help separate these from precision approaches.
    • LNAV - Lateral Navigation (uses an MDA)
    • LNAV/VNAV - Lateral Navigation/Vertical Navigation (uses a DA)
    • LP - Localizer Performance w/o Vertical Guidance (uses an MDA)
    • LPV - Localizer Performance w/ Vertical Guidance (uses a DA)
    • RNP, written "RNAV (RNP)", see below
  • RNP approach (FAA page)
    • When you see RNP in the approach label, it can be interpreted as 'authorization required' because in reality, any RNAV has some RNP.
  • Segments:
    • What is considered the Initial Approach Segment on an approach?
      • The initial approach segment begins at the initial approach fix and ends where it joins the intermediate approach segment.
    • What is considered to be the Intermediate Approach Segment?
      • The intermediate segment (normally aligned within 30 degrees of the runway) begins at the intermediate point and ends at the beginning of the final approach course.
    • What is the Final Approach Segment?
      • The final approach segment for a precision approach begins where the glide slope is intercepted at the minimum glide slope intercept altitude shown on the approach chart;
      • The final approach segment for a non-precision approach begins at either a designated Final Approach Fix (FAF) or at the point where you are established on the final approach course.
      • When the FAF is not designated, such as where there is a VOR or NDB on the field of intended landing as published, the Final Approach Point (FAP) is where the procedure turn intersects the final approach course inbound.
    • What is considered the Missed Approach Segment?
      • The missed approach segment begins at the MAP and ends at a designated point.
  • Understanding GPS approaches (Pilot Workshop)
  • AIM: cross DER > 35’, 400’ before first turn, 200 FPNM until minimum IFR altitude*
    • 200 FPNM = 233 FPM @ 60 GS, 267 @ 80, 300 @ 90
    • * unless specified different (crossing alt, DP) turn @ higher altitude or @ fix
  • DPs and ODPs (ODPs are normally narrative)
    • Obstacles w/in 1nm & < 200’ tall are “low close-in obstacles” and are generally NOT factored in ODP
  • Minimum takeoff WX?  There isn’t, but a technique is highest published circling mins 
- FIG -

Wednesday, April 22, 2020

Engines / Powerplants

  • Aspirated ROT:
    • Engine will lose about 3% of its power for every 1,000’ of altitude
  • CHT (cylinder head temperature)
    • If too high > Enrich mixture / decrease pitch attitude / reduce power / open cowl flaps
    • Shock cooling > avoid it by allowing CHT to drop slowly
  • EGT (exhaust gas temperature)
    • Lean to 100°F on rich side of peak EGT for best operation (technique)
  • Leaning
    • Whenever mixture is adjusted, rich or lean, it should be done slowly.
    • Use full rich mixture during take-off or climb below 5K density altitude (Lycoming).
    • Anytime power setting is 75% or less at any altitude
    • At high altitude airports (> 5K), lean for taxi, take-off, traffic pattern entry and landing. 
    • Landing at airports < 5K density altitude, adjust mixture for descent, but only as required.  You can't go wrong if you keep the engine running smoothly!
    • Before entering pattern, go full rich
    • Methods:
      • Fixed props: gradually lean mixture until either tachometer or airspeed peaks.
        • From a 1969 172 POH, “…the mixture should be leaded as follows:  pull mixture control out until engine RPM peaks and begins to fall off, then enrichen slightly back to peak RPM.
      • Controllable pitch props: lean until a slight increase of airspeed is noted.
        • Slowly lean mixture until engine becomes rough or until power rapidly diminishes as noted by undesirable decrease in airspeed.  When either occurs, enrich mixture to obtain an evenly firing engine or to regain most of the lost airspeed or engine RPM.
        • When leaned, roughness is caused by misfiring due to a mixture which can’t support combustion.  It’s eliminated by slightly enriching mixture.
    • With EGT: lean to 100°F on rich side of peak EGT for best operation.
    • At all times, caution must be taken not to shock cool the cylinders.  Maximum recommended temperature change should not exceed 50°F per minute.
    • FAA on leaning 
    • Lycoming on leaning 
  • Oil
    • Too warm > Enrich mixture / decrease pitch attitude / reduce power / open cowl flaps
  • Percent power RPM:  see separate post
- FIG -

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