Friday, February 10, 2017

The Dreaded Visual

Ask a first time jet pilot half-way through their simulator training what they find to be the most difficult approach to fly, and you may get one of several answers.  A single-engine hand-flown ILS, a circle to land, or maybe a full NDB approach- depending on the aircraft in question and the pilot' s strengths and weaknesses, any of a number of approaches are likely to prove most vexing.

Ask that same pilot the same question when he's just begun mentoring, and it's nearly certain the answer will be the effect of, "I just can't nail that visual approach yet."  I can hear the laughing from piston pilots, "What could be easier than a visual?"  Yet every pilot I've mentored has struggled with mastering visuals long after they could fly a single engine ILS while reciting poetry.

Why?  It's largely an unfortunate by-product of one of the factors that makes flying jets so safe- how tightly scripted most operations are.  On a nice ILS with a six to eight mile final, a properly trained pilot knows exactly when to be at "X" speed, and have "Y" configuration of flap and gear extension.  There's a recipe cooked up by the aircraft manufacture for optimal flight- follow the recipe and the result will be pleasing every time.

In contrast, a visual approach can feel like trying to whip up a gourmet dinner with the dredges of a mostly empty pantry- not a lot of guidance, and often nothing good to work with.  Being cleared for the visual from 5000' AFL to a runway with no instrument approach (while eight miles out on base leg) requires creativity and improvisation that isn't practiced much, if at all, in simulator training.

There are some tips, however, a pilot can heed to make the visual work.  First is recognizing that most new pilots in this situation will find themselves on short final with an excess, not a shortage, of energy.  It is much harder to take energy, either in the form of airspeed or altitude, out of a jet aircraft than a prop.  For this reason, more often than not, the first reaction to being cleared for a visual approach should be to bring the thrust levers to idle.  Slowing the aircraft so that landing gear and at least partial flaps can be extended results in a much more draggy and easy to manage aircraft.  Thrust can always be added back if it turns out the energy state isn't excessive.

Along with carrying too much energy, another common mistake is too quickly disengaging the autopilot.  Because the approach doesn't fall into one of the typical scenarios practiced in sim training, the pilot has a reflexive reaction to return to basics and hand-fly.  For an inexperienced pilot this greatly increases the workload, and usually decreases the quality of the approach.  Using the autopilot in the basic heading and vertical speed modes will let the pilot focus on keeping the big picture looking good.

Finally, just because the approach is a visual, doesn't mean electronic guidance shouldn't be used.  If the runway has an IFR approach, it should be used for reference.  If not, creative use of GPS or FMS can give, at a minimum, lateral guidance to any runway, anywhere.  Used properly, synthetic vision can provide accurate vertical guidance to a runway with no VASI or approach.  While these tricks aren't always taught in initial sim training, by expending the effort to learn them, a pilot will find visuals much less taxing.

Saturday, November 22, 2014

Concrete Runways and Rain

Two recent Phenom overruns have an interesting variable in common: both occurred landing on a wet, concrete, ungrooved runway.  Ungrooved concrete runways represent less than one in five of all paved runways in the US, but can be particularly treacherous when wet.

The graphic below comes from a nearly 50 year old study conducted by NASA.  The aircraft in question had a higher touchdown speed than would be common for a business jet, but its absence of speedbrakes, spoilers, or reverse thrust, and the fact it has a single main wheel on each assembly compare to many light jets.  Notice the massive difference in stopping distance required on an ungrooved concrete runway, versus a grooved asphalt runway.

Taking into account that brakes were not engaged until approximately 1700' from the runway threshold, the ground roll portion of a landing on wet ungrooved concrete was 250%-290% that of a dry surface (depending on surface texture of the concrete, as represented by surface "A" and "D" above).  A wet, grooved, asphalt surface, in contrast, required only 3% more stopping surface than a dry runway.

Thursday, July 24, 2014

Use Vertical Guidance on a Non-WAAS GPS Approach

When IFR GPS broke onto the scene nearly two decades ago, it caused a sea change in the way approaches could be flown into airports too small to have an ILS. Rather than conducting an approach referencing a relatively inaccurate VOR or NDB signal, every airport in the country could be approached with localizer accuracy. What the first GPS did for lateral navigation, WAAS has done for vertical navigation. Yet some pilots still aren’t taking full advantage of WAAS capability, and some training providers are teaching procedures that hobble a major safety benefit that WAAS presents.

The issue is the ability of WAAS-capable Garmin navigators to create vertical guidance as part of most GPS approaches without published WAAS minimums. This advisory vertical guidance turns an LNAV-only approach into an "LNAV+V" approach. Where "+V" vertical guidance is available, the autopilot is fully capable of coupling to the vertical path, as if the approach was a full-blown LPV or ILS.

Yet many pilots conduct this type of approach in the same manner they would a VOR or NDB, setting the MDA in the altitude selector and using vertical speed mode to reach MDA. It is true that the presence of vertical guidance on an LNAV approach doesn’t change the MDA, nor make it an official APV (approach with vertical guidance) in the eyes of the FAA. However, there’s no reason not to use the same procedure to fly the approach as if it were an LPV, coupling vertically via the GP (glide path) mode, and never leveling off at MDA.

Flying an LNAV+V in the manner of a precision, rather than non-precision, approach offers several safety benefits. The most important is that the aircraft can be configured to land and be on-speed outside the FAF, with no further configuration or power changes required. Wherever the pilot may be when visual contact with the runway is acquired, the aircraft will be on a stable path to the touchdown zone.

In contrast, a pilot who dives down to MDA at a high rate of descent may visually acquire the runway before the aircraft is in a position to begin further descent. Deciding when it’s safe to leave MDA can require quick thinking at a time when the aircraft is very close to terrain –– not a desirable situation.

Another benefit is that, as level-off at MDA is not required, the altitude pre-selector can be set to the level-off altitude of the missed approach once the FAF is passed. Forgetting to set missed approach altitude after level-off at MDA is one of the most common errors made by transitioning jet pilots, and beginning the missed without the correct altitude set can lead to undesirable autopilot behavior or an ATC violation.

You can employ a trick to turn an MDA into a DA. Built into a published DA is consideration that if a pilot looks up at DA, the aircraft will still be traveling downwards while the pilot makes the decision to go missed, adds power, and transitions to a climb. An MDA, however, is exactly what it says –– a minimum altitude below which the pilot must not go unless landing. The solution is simple: add "padding" to the MDA so that even if it takes a few seconds to get the plane climbing, MDA will not be violated.

A light jet flying on a three-degree descent path will be descending about 550 FPM, or a little over 9 feet per second. Assuming a worst-case scenario of five seconds to transition from descent to climb and rounding the result up, you get 50 feet. So, adding 50 feet to the published MDA results in the new functional DA –– the altitude at which the missed approach will commence, and which should be bugged as "minimums."

Wednesday, April 16, 2014

Understanding V2

Many of the new V-speeds first time jet pilots are exposed to are only used in the event of an engine failure.  V2, VFS, VENR, VAC- depending on the aircraft manufacturer and phase of flight (takeoff, go-around, etc.), a pilot will calculate several speeds for each takeoff and landing that he will almost certainly not need to use.  While most pilots quickly learn to compute the speeds in initial training, and use them nearly every simulator session during one simulated engine failure after another, many pilots harbor misunderstandings about what the speeds represent.

V2, for example, is often mistakenly thought to be the jet equivalent of VXSE, or single engine best angle of climb speed.  Pilots of piston and turbo-prop multi-engine aircraft learn not only the speeds for best rate and angle of climb when both engines are working, VY and VX respectively, but those for best rate and angle with one engine failed.  For most non-jet twins, these speeds are given for one weight and altitude combination, and the pilot never ventures beyond these default values.

In contrast, before every takeoff in a jet, the pilot will typically calculate four V-speeds based on criteria such as weight, altitude, temperature, and flap settings.  The first two of these speeds, V1 and VR, determine how late into the takeoff a pilot may abort the takeoff, and when the pilot begins the transition to flight.  The other two speeds, V2 and a "final" climb speed (which goes by different names depending on the aircraft manufacturer- we'll call it VFINAL), are only used if an engine fails during the takeoff, after the point at which an abort can be safely completed.

During initial sim training, for nearly every simulated engine failure on takeoff, the pilot is presented with a textbook, that is to say, worst-case, event.  The engine is programmed to fail just before V1, the pilot reacts, rotates the aircraft at VR, and climbs to a safe altitude above obstructions at V2.  Once above immediately threatening obstacles, the pilot accelerates to the final climb speed and retracts flaps.  From this sequence many pilots develop a logical, but fallacious, correlation of V2 to VXSE, and VFINAL to VYSE.

In fact, it is VFINAL that correlates much more closely to VXSE.  So what does V2 represent then? In short, a compromise.  For most circumstances a jet will encounter, certification requirements define takeoff distance as the longer of the distance to either bring an airplane up to V1 and initiate an aborted takeoff, or continue on one engine so that the aircraft reaches V2 speed at 35' AGL.  Clearly the higher the V2 speed defined by the manufacturer, the longer the distance that will necessary to accelerate from V1 to V2 on only one engine.  Due to the desire to minimize published takeoff distances, the manufacturer often sets V2 to be the minimum allowed by certification requirements, based on minimum allowable ratios of V2 to stall speed and VMC. 

So what's the implication of V2 being a bit lower than VXSE? A brief review of basic aerodynamics will illustrate the drawback to this minimum V2 approach.  While rate of climb is determined by the excess power an aircraft has available, angle of climb is determined by the amount of excess thrust available.  For given atmospheric conditions, the thrust output of a jet engine is nearly constant if plotted against airspeed, while the thrust required by the aircraft follows the familiar "J" curve.  Given these two curves, it is apparent that the greatest distance between the two lines, or the point of maximum excess thrust, occurs at minimum thrust required speed, which is to say, the minimum drag point of the curve.

At any speed slower than this point, the increase in induced drag means that climb angle will suffer.  So by selecting a V2 which lies below minimum drag speed, the aircraft manufacture is giving up some possible climb gradient in favor of a reduced takeoff distance.  As most jet aircraft have an abundance of extra thrust, this is an acceptable tradeoff, and even during a non-optimal climb at V2, most jets will turn out adequate, or even impressive, single-engine climb angles.

While not common in light jets, some larger jet aircraft have software or performance charts that allow for an optimized, rather than minimum, V2.  If takeoff distance is calculated as 4000', but 9000' of runway is available, it is easy to see that by increasing the V speeds until optimal V2 is reached, extra runway can be converted into better engine-out climb performance.

What is common in light jets is the ability to depart with a reduced, or even zero, flap setting when runway available is not a limit, but engine-out climb performance may be.  By departing with reduced flaps, the aircraft must accelerate to a higher speed before rotation, which brings the aircraft closer to true VXSE.  Also, the reduced drag of a lower flap extension means that the thrust-required curve shifts down- for any given speed less thrust is required simply to maintain level flight, so more thrust is excess, and can contribute to a relatively greater climb angle.

Beyond the performance implications of knowing what V2 really represents, there is also a practical flight application.  While nearly all engine failures in the simulator occur just prior to V1, a real engine failure can occur at any point in the takeoff roll.  When I conduct in-aircraft training, for example, I typically retard one thrust lever to idle just as VR is reached.  With the two to three second spool down time of a jet engine, by the time the pilot recognizes the engine has failed, the airspeed is often five to ten knots above V2.   Most pilots just out of sim training exhibit a strong desire to pitch the aircraft up until V2 is reached, then hold V2 until clean up altitude.

Keeping in mind the drag curve, it becomes apparent that a pilot who has attained a speed higher than V2 before recognizing an engine failure would be better served by maintaining that higher speed until at clean up altitude, provided the aircraft hasn't accelerated so much as to be faster than VFINAL.  Doing so will result in lower drag, and thus a better climb angle.

Tuesday, May 7, 2013

Understanding V1

One of the many things a first time jet pilot learns in training is a set of entirely new speeds- V1 , V2, VREF just being a few.  To make the issue even more complex, different manufactures sometimes call the same speed by different names; what Embraer calls VFS, Cessna calls VENR.  Most pilots struggle through these new performance considerations along with a raft of other new topics, and understandably often come out with a shaky grasp of just what exactly these new speeds represent.

V1, for example, is the very first “V” speed a jet will encounter as it transitions to flight, yet most pilots have an erroneous picture of what this critical speed represents.  Ask the average jet pilot to define V1, and they will respond along the lines of, “V1 is takeoff decision speed- the highest speed at which the plane can have an engine fail during takeoff and abort on the runway in the space calculated, or continue the takeoff on one engine.”  This statement, besides being almost universally believed, is completely false.

The critical fallacy lies in the assumption that V1 represents a decision speed of sorts.  Most pilots believe that if an engine fails at V1, the pilot is given a space of time (three seconds is often claimed) to recognize the failure, then begin the process of aborting the takeoff- bringing the thrust levers to idle, initiating braking, deploying speed-brakes or spoilers.  What the FAA defines V1 to be is very different.

FAR 1.2 defines all the V speeds a pilot could ever use.  Telling about the complexity of V1 is that while most speeds are defined in just a few words- “VR means rotation speed”, “V2 means takeoff safety speed”, the definition for V1 runs for two lengthy sentences.  That definition reads, in part, “… the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance…”  Clearly if the pilot must take the first action at V1 in order to stop in the calculated distance, the decision to abort must have been made well before V1.  It also follows that if an engine fails at V1, there is no way a pilot could instantaneously recognize the failure and initiate the abort.

Indeed, the highest speed at which an engine can fail on takeoff and still allow for an abort within accelerate-stop distance is not V1 at all, but a speed most pilots have never heard of- VEF, or engine failure speed.  VEF is not published by aircraft manufacturers, but rather is used during the testing that determines the V speeds which are published.  The most critical engine is failed at VEF and the airplane continues to accelerate on one engine while the pilot recognizes that the engine has failed and makes the decision to abort.  Once the test pilot initiates the abort, that speed is noted and becomes V1.

The practical implication of this distinction is that once V1 has been reached, it is too late to initiate an abort within the runway distance calculated.  Unfortunately, as VEF is not published, it is impossible to know exactly when the moment passes which would allow for a successful abort.  For this reason, many operators call “V1” aloud at 5 knots before V1, not at V1 itself.  Once the flying pilot hears “V1” spoken, the hand on the thrust levers is moved to the control yoke, signifying that from that point on, no matter what occurs the plane will be taken airborne.

Important also to understand is that during the aircraft testing for calculation of accelerate-stop distances, the speed never gets above V1.  Every second that an airplane is accelerating beyond V1, the energy the brakes would be required to dissipate during an abort increases as a function of the square of the speed increase.  Putting some numbers to this abstraction, consider the case of a light jet departing at maximum takeoff weight, with a calculated V1 of 105 knots. 

Just as V1 is reached, and the pilot erroneously thinks he has his last chance to decide to abort, a loud bang is heard, and the plane pulls to one side.  Unfortunately for the pilot, rather than the perceived engine failure, what has actually occurred is a tire blow-out.   During the few seconds it takes the pilot to react both engines, not one, are producing takeoff thrust, and continuing to accelerate the aircraft past V1.  If the plane reaches a peak speed of only ten knots beyond V1, the brakes must now dissipate 20% more energy than had the abort been initiated at V1.  Beyond the fact that there is no certification requirement for the brakes to be able absorb any energy beyond that existing at the highest weight and V1 combination demonstrated, there is also no performance data to know how much runway would be needed even if the brakes are able to handle the extra energy.  Adding to the chaos, one brake must now absorb all the energy of the aircraft, as the blown tire’s brake has been rendered useless.

Unfortunately many rejected takeoff (RTO) accidents have been caused by such a set of circumstances.  One industry study found that 80% of RTO accidents were avoidable, and the accident planes would have been able to safely continue the takeoff with the problem in effect.  For this reason, most jet operators use a two-phase abort decision tree.   Up to a specified airspeed, usually 70 knots for light jets, the takeoff can be aborted for any reason.  Above this speed and before V1, if the runway available is less than the runway required plus a defined safety margin (often 50%), the takeoff will only be aborted for an engine failure, engine fire, or the perception that the aircraft is unable to fly.

When there’s a loud bump right at V1, a pilot has no way of knowing for sure what has occurred, and what the safest course to take is.  But history and certification requirements point to continuing the takeoff as the path most likely to lead to a happy outcome.

Wednesday, December 21, 2011

Lower Mins with Synthetic Vision?

You can find all sort of interesting things on approach plates.  Take the ILS 17L into Colorado Springs (KCOS).  The approach is a standard Category I ILS, yet the minimums block contains some non-standard minimums:

What’s that “RA 169” all about?

RA in this case stands for radar altimeter; the decision to go-around or land is made referenced to height above ground, not a baro-altimeter setting.  Most modern light jets equipped with radar altimeters have a means of selecting “Baro” or “Radar” minimums to be entered into the PFD, triggering the “minimums” alert.  Looking at notes 1 and 2, though, it’s apparent that not everyone can use a radar altitiude of 169’ as DH.  The big limit is the note that use of a heads-up display, or HUD, is required.  Even aircraft equipped with a HUD aren’t automatically able to drop an extra 50’; the flight crew and aircraft must have been granted authority via an LOA, as well.

The requirements to be granted the LOA are spelled out by the FAA in order 8400.13D, which specifies the crew and aircraft must be certified for Category II operations.  Additionally, single pilot operations are specifically prohibited from using the lower minimums.

So while it’s apparent the average owner –flown light jet won’t be able to fly Cat I ILSs to 150’ minimums anytime soon, what is of interest is the trend approaches like this represent.  “Special Authorization (SA)” procedures are becoming more common, with required navigation performance (RNP) approaches, being the main example.  Rather than a one-size fits all mentality to airspace management, the FAA is shifting to a “best equipped, best served” philosophy.  Operators with the requisite toys will be authorized to fly approaches others can’t, or to fly a standard approach to lower minimums.

And while it will likely be some time before light jets are delivered with HUD units, the next SA ILS authorization being studied will utilize a technology already available- synthetic vision (SV).  As envisioned, aircraft with a radar altimeter, WAAS, and SV will also be able to take advantage of 150’ DHs.  Pilot and aircraft certification will still be required, and it’s not certain if single pilot operations will be permitted.

Sunday, May 15, 2011

Nice to be SOL

WAAS is unquestionably a revolutionary technology for the IFR pilot.  With no ground infrastructure at all, we can now be guided as low as an ILS to any piece of pavement at any airport.  Even airports without a WAAS approach benefit as the vertical accuracy of WAAS allows for an advisory glidepath to MDA on plain-vanilla GPS approaches.

Well, to airports in North America, anyhow.  The biggest limit of WAAS is that as it depends on geo-stationary satellites, the coverage area is limited to the parts of the planet over which those satellites “hover”.  Travel outside these areas and GPS technology steps back to the pre-WAAS era.

That’s why the recent declaration of the EGNOS systems as safety of life (SOL) capable is great news.  EGNOS, or European Geostationary Navigation Overlay Service, is the European version of WAAS, both of which are generically called Space Based Augmentation Systems (SBAS).

EGNOS covers most of Europe

EGNOS has been operational for several years, and in March was declared ready for critical SOL operations, such as aviation.  Within a week a Falcon 900 flew the first LPV approach in Europe, to Pau Pyrénées in the south of France.
Garmin has already upgraded the G1000 to be capable of using EGNOS guidance.  When the navigation database cycle 1101 was installed in G1000 systems in early January, it also unlocked EGNOS capability.  Pressing the SBAS soft key on the GPS status page now shows EGNOS in addition to WAAS and MSAS (the Japanese version).

G1000 is now EGNOS ready

Unfortunately it will be some time before full benefit of EGNOS is realized.  To start, not very many airports in Europe have approaches with LNAV/ VNAV or LPV minimums, with only France and Germany having any in large numbers.   Further, even those that do may prohibit use of EGNOS until flight checking is complete.  A recent trip into Hamburg, Germany (EDDH) turned up the following NOTAM:

“Use of SBAS- (EGNOS-) signals for APV- Baro VNAV operations prohibited until further notice.  Use of LNAV/ VNAV minima with a certified Baro VNAV system…still permitted.”

So while LNAV/ VNAV minimums are published to EDDH, they can’t be flown yet via EGNOS.  This will change with time, and European operators will enjoy the benefits of SBAS approach aiding we’ve enjoyed in the US for several years.