Understanding V1

One of the many things a first time jet pilot learns in training is a set of entirely new speeds- V₁ , V₂, V_REF just being a few.  To make the issue even more complex, different manufactures sometimes call the same speed by different names; what Embraer calls V_FS, Cessna calls V_ENR.  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- “V_R 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- V_EF, or engine failure speed.  V_EF 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 V_EF 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 V_EF 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.

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.

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.

SVS, huh? What is it good for?

“I don’t fly in the mountains, so I really don’t need synthetic vision.”

If I had a dollar…Yet I can’t blame the average pilot who has yet to experience SV for sharing this opinion.  The marketing materials of avionics manufactures are full of scary pictures of red and yellow mountain tops depicted on the PFD.  That or a low pass over a big cell tower.  Presented thus, it’s easy to see synthetic vision (SV) as solely a tool to avoid hitting things.  Yet even if you fly in the flatlands, SV is one heck of a cool feature to have.  Here’s why.

Let’s start with a tour of some recent avionics innovations.  When the first IFR GPS units were certified I remember flying an approach and thinking “This is great- every approach can now be flown like a Localizer approach.”  Flash forward a few years to WAAS- now every approach can be flown like an ILS, even better!  Well- what’s even easier than an ILS? A visual, of course, and SV had moved the state of the art to where every approach can encompass the same instant situation awareness experienced on a visual.  Of course, the published procedure must still be followed, but seeing the runway symbol and instantaneous flight path marker (FPM) crisply drawn on the PFD means the same intuitive techniques used to line up with a runway on a clear VMC day can be used when it’s 200-and-a-half.

For example, imagine a descent is being made to MDA on a non-precision approach.  No vertical guidance is provided, so the pilot has no choice but to dive to MDA, right? Not with SV- by controlling rate of descent so that the runway symbol stays at the minus 3° pitch point of the PFD, the plane is descending as smoothly on a 3° path as if on a glideslope.

Caption: Runway at three degrees and flight path marker on runway= stable approach

Another time SV can be a great aid to stabilized approaches is when cleared for a visual approach to an unfamiliar airport from a base leg entry.  These can be difficult to plan correctly, as the visual cues are difficult to interpret at a ninety degree angle to the runway.  Often pilots will turn final and find themselves quite a bit higher than anticipated.

If the runway has an approach with vertical guidance published, rolling out right on glidepath is child’s play.  By loading the approach and activating the leg to the runway threshold, the pathway boxes will be visible in profile.  They will be descending to the runway on roughly a 3° angle, so steering the FPM so that it lies over any pathway box will ensure the aircraft is descending so as to perfectly intercept the final on glidepath.

Caption: “Gunsighting” pathway box with FPM to ensure rolling out on glidepath

Y or Z, Part II

Last post I looked at the situation where a non-standard missed approach gradient could cause two sets of minimums to be needed for the same approach, thus resulting in two versions of the approach, a “Y” and a “Z”.  I mentioned there are three reasons two approaches of the same type could exist to the same runway, let’s look at the other two now- both involve RNAV approaches.

The second case is where two RNAV (GPS) approaches exist to the same runway.  Typical of this example is Burlington, VT, with an RNAV (GPS) Y and Z approach to runway 15.  The Z approach has published WAAS (or FMS) minimums of LNAV/VNAV to 660’ and LNAV-only mins of 940’.  The Y approach only has LNAV mins published, but to 760’, lower than the LNAV mins for the Z approach.   A quick view of the profile view shows why; the Y approach has a stepdown waypoint, JUNEL, abeam a 639’ high tower.  So an aircraft not equipped with WAAS/ FMS would want to execute the Y approach, one capable of LNAV/VNAV approaches would want the Z approach.

BTV RNAV (GPS) Y RWY 15

 BTV RNAV (GPS) Z RWY 15

Sometimes the approach course is completely different, as is the case with the RNAV (GPS) Y or Z --- at Half Moon Bay, CA.  The Y approach has LNAV only minimums, and is flown mostly over the water, with two doglegs, or course changes.  The Z approach takes advantage of LPV’s tighter guidance to get down twice as low as the Y, and is aligned with the runway from a 14 mile final.

HAF RNAV (GPS) Y RWY 30

 HAF RNAV (GPS) Z RWY 30

The other case where multiple RNAV approaches are published to the same runway involves a new type of approach- the required navigation performance, or RNP approach.  RNP approaches are the approach of the future, and represent a shift from navaid- based approaches to performance-based approaches.  In other words, as opposed to a VOR approach, which can only be flown by reference to the designated VOR, an RNP approach only requires that an aircraft be able to maintain a specified level of accuracy and integrity.  For all practical purposes, RNP approaches now require GPS, but the option in the future to rely on other sensors exists.

An RNP approach is an RNAV approach in that it depends on the ability to navigate to any arbitrary point in space.  Yet its equipment requirements go beyond an IFR GPS unit, so it is classified RNAV (RNP), as opposed to RNAV (GPS).  A way is needed to differentiate between these multiple RNAV approaches, so again X, Y, and Z come into play.  An example is San Francisco, with an RNAV (GPS) Z RWY 28R, and an RNAV (RNP) Y RWY 28R approach.

We’ll look more at RNP approaches in the future, but for now there aren’t many GA aircraft capable of flying them.  Only the flagship products from the largest business jet manufactures have received approval, and individual operators and pilots need authorization to fly even an approved aircraft on these approaches.