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Enroute adjustments

Revision: March 14, 2012, at 12:25 PM

Module content

Enroute navigation requires the pilot to continually monitor progress against plan; heading adjustments may be needed to maintain the planned route. If progress is slower than planned then diversion to an alternate airfield may be necessary and there are standard procedures to be implemented when uncertain of position.

Monitoring and recording flight progress

Monitoring involves checking from clock to map to ground anticipating what should be in view a few minutes ahead plus intermittent position fixing to establish the track made good, estimate track error and actual ground speed. The essential navigation instruments are just the compass and the clock: to provide direction and ground speed.

Deviations from the track required occur because:

  • the pilot is not maintaining the planned heading or has set the wrong heading, for example the heading for another leg
  • unrecognised compass error [deviation] causes the heading flown not to be the planned magnetic heading
  • the wind velocity is substantially different from that used for the flight plan or it was applied incorrectly during flight planning
  • the required track direction was incorrectly measured or converted to magnetic during flight plan preparation.

Position fixing

There are basically two methods of fixing the aircraft's position. The first, and the most common in light aircraft navigation, is by identifying a landmark close to the aircraft. The second is by identifying (or establishing) two, or more, lines of position [LOP] such that their point of intersection provides the position fix. A line of position is a line drawn, or already existing, on the chart indicating that the aircraft's position is somewhere along it. Note that there should be a reasonable angular difference, maybe more than 30, between two LOPs in order to derive a useful position fix.

For flying in New Zealand, with plenty of topographical features (coastline, rivers, road, rail, towns, transmission lines, ridges and peaks), position fixing by landmark identification is quite practical.

Setting compass heading

Most compasses will have differences between the indicated compass heading and actual magnetic heading, due to internal and external magnetic interference. This is the deviation. If the deviation has been measured ther may be a compass correction card displayed in the aircraft. Such a card might look like this:

Compass correction card
Heading magnetic045090135180225270315360
Compass correction+2+5+20-2-5-20

To find the heading to set on the compass just add the deviation value to, or subtract it from the magnetic heading.

Track error adjustments

At any time after departure, when the aircraft's position has been pinpointed and found to be off track, heading adjustments will be necessary: initially to regain the required track and then to maintain it or, alternatively, for a new heading to track directly to the next turning point.

In most cases of pilotage, where track errors are immediately obvious, a simple heading correction will bring you back on track.

Using the 1-in-60 rule

The 1-in-60 rule of thumb can be used to determine track error, given distance travelled and distance off track.

Having pinpointed the aircraft's position, estimate the distance off track (DO) and the distance travelled (DT) along the leg. The track error = DO/DT 60.

If you are less than halfway along the leg, simply apply double the track error (in the correct direction) to converge onto track before reaching your destination.

If more than halfway along the leg, track direct to the next waypoint by calculating the closing angle using the distance off track [DO] and the distance to go [DTG]- closing angle = DO/DTG 60.

Recalculating ETE/ETA and fuel consumption

Being off-track, because of a variation in wind, is much more likely to be noticed quickly than if on track but adversely affected by a stronger than expected headwind, or tailwind. Whenever a position fix is obtained, whether it is on-track or off-track, it is advisable to calculate the ground speed achieved and to re-estimate the ETE for the leg, ETA at the next waypoint and the destination.

Ground speed [GS] in knots is distance travelled [DT] / elapsed time in minutes [ET] 60
and
Time in minutes to the next waypoint = distance to go [DTG] / GS 60

Checking fuel consumption

Fuel flow indications may be monitored for abnormalities as part of the continuing in-flight instrument scan, however a calculation of consumption rate should be made at half-hourly or hourly intervals to check for any significant variation from the hourly consumption rate used in the flight plan. Fuel consumption should always be measured in terms of time not distance.

Diverting to an alternate airfield

During flight the pilot should always be aware of the general direction of the planned alternate airfields so that, should a diversion be deemed necessary the aircraft can be headed in the general direction of the selected alternate without unnecessary delay. The mental calculations required to refine the heading, estimate distance, ETA and fuel requirement are then done without wasting time and fuel.

Line of sight distance and landmarks

Knowledge and use of landmarks is an essential part of ultralight pilotage, thus on cross-country flights it is useful to know at what distance any landmark, particularly those distinguished by height and shape, might be discernible. The rule-of-thumb is, given unlimited visibility and an eagle eye, the maximum optical line of sight (LOS) distance in nautical miles is equal to the square root of the observer's height in feet. VHF transmissions are also LOS.

Theoretical LOS distance to horizon
Observer height (feet)Maximum LOS distance (NM)
103.2
10010
100032
10,000100

The theoretical distance, in nautical miles, at which a landmark may be seen is near enough to the sum of the square root of the height of the top of the landmark [in feet] and the square root of the observer's height. Theoretically then a pilot flying at 10,000 feet might first see the highest point of an island, with an elevation of 1000 feet, from 132 nm away [100 + 32].

Lost procedure

There are occasions during a cross country flight when the pilot is uncertain about the aircraft's position, particularly when there are considerable distances between verifiable landmarks and a near-track landmark has not come into view . However if proper flight planning and checking procedures are followed and actual vs planned flight progress is continually monitored and recorded then probably the only way to become really lost in clement weather and reasonable visibility is if an enroute heading adjustment is incorrectly calculated or implemented, or if a turning point is overflown without noticing.

There are a few rules which must be followed if thought to be lost or caught in a difficult situation:

  • Fly the aeroplane! You must not concentrate all attention on the navigation problem, keep the normal scan going otherwise you can readily lose control of the aircraft.
  • If the ETA at the next waypoint has not yet, or only recently, lapsed then hold the heading resist the temptation to start wandering about searching for landmarks.
  • However if the ETA at the waypoint has significantly passed then choose a landmark below the aircraft, log the time and then orbit the landmark while you carry out a quick recheck of the running log and previous mental DR and start the procedure detailed in the next paragraph, but don't forget rule 1 Fly the aeroplane!. There is no point in wasting fuel while doing this so reduce power, and airspeed, to the best endurance setting for a safe flight speed. If no obvious error is found which will provide the basis for a position estimate then:
  • If below 3000 feet AGL then climb a little, cloud base permitting. The theoretical line of sight distance at 4000 feet AGL is 65 NM all round which provides sufficient coverage for the 'Mk1 eyeball' to pick up all the major landmarks, near and middle distance, which aren't concealed by terrain or atmospheric conditions. If climbing takes you above an inversion layer you may find surface visibility is better just below the inversion. Remember that on a bright day scattered cloud shadows may make some landmarks difficult to pick up even if relatively close. Reduce power back to best endurance.
  • Read from ground to map! Normally in flight the navigator should be continually identifying features on the map and waiting for the next one to come up on track, within an estimated time. When uncertain of position the procedure is reversed, look for two or more large features on the ground and then identify features on the chart that are in the same juxtaposition. Prominent line features are best although, quite often, a spot feature is easily identified. If you see a prominent line feature, then fly along it until you can derive a fix from an intersect or a verifiable landmark.
  • Don't stay up too late! Be prepared to make a precautionary landing well before the fuel content reaches the 30 minute reserve figure and well before oncoming twilight reduces visibility at ground level. You need to ensure that a precautionary landing isn't downgraded to a forced landing because of fuel exhaustion. Try to select a suitable site near a house, remember after you have landed you still have to secure the aircraft, protect it from stock and perhaps get some help very difficult in the bush and in the dark!

There are many circumstances where a precautionary landing is a wise move, among them:

  • Deteriorating weather
  • Oncoming darkness
  • Fuel reaching reserve level
  • Lost and you decide to obtain help on the ground
  • Engine rough running [although this might be considered a forced landing]
  • Occupant illness

The technique for precautionary landings at other than a prepared landing ground is essentially the same as that for short field landings except that additional low level passes should be made to check the hazards with particular care in wire location and avoidance. Map out the landing/run-out path and also determine the escape route in the event of an aborted landing.

And lastly

  • Communicate! Share the problem.

Dangers of flight into cloud or when lacking visual references

The human vestibular system

When walking, a person's prime sense of orientation is provided by visual references but if vision is severely degraded the vestibular system in the inner ears, which senses motion and gravity (thus roll, pitch and yaw), generally allows us to keep our balance. However the vestibular system is not designed for high speed or angular motion and cannot be used as an inflight back-up system, i.e. you cannot close your eyes and continue to fly straight and level. Motion of the fluid within the ears' semicircular canals is affected by inertia and will feed quite erroneous prompts to the brain resulting in various types and levels of vertigo.

For example without the external visual references of clear sky, terrain or a horizon forward deceleration tends to impart a pitching down sensation whilst forward acceleration gives a pitching up sensation. Once settled into a constant rate turn the sensation is of not turning at all, but when the turn is halted the sensation is then of turning in the opposite direction. In addition the vestibular system will not detect slow rates of bank so that if the aircraft is banking at the rate of one or two degrees per second the vestibular system won't send any prompts to the brain, which will consider the aircraft is still flying straight and level, while any associated speed changes may provide contrary sensations. For example if the aircraft is slowly banking and accelerating in a descending turn the sensation may well be one of pitching up. Such sensations disorient the pilot.

Spatial disorientation

Aircraft accidents caused by spatial disorientation are usually fatal and occur when VFR flight is continued in adverse visibility conditions cloud, fog, smoke, haze, showers, darkness and combinations thereof. Pilots who have not been trained to fly solely by visual reference to the indicators in a specialised "blind flying" panel/display will soon find themselves experiencing spatial disorientation should they, inadvertently or deliberately, enter instrument meteorological conditions [IMC] where the external visual references by which they normally orient themselves in visual meteorological conditions are lost. The same applies to any atmospheric condition where the visual references horizon [principally], terrain and clear sky are lost or just significantly reduced, see white-out/flat light for example.

Thus a non-instrument rated pilot would be unable to maintain controlled flight in cloud, or maybe even in conditions where the horizon disappears, and even an instrument rated pilot cannot fly in cloud without the minimum IFR instrumentation. Nor can an instrument rated pilot in an IFR aircraft fly where the aircraft can't outclimb rising terrain, whether it is concealed or not; in addition many horrific accidents have occurred when an IFR pilot has descended below the area 'lowest safe altitude' in IMC and impacted the terrain; such events are classified as controlled flight into terrain.

Note: even a pilot who is well experienced in flying in instrument meteorological conditions may occasionally experience a phenomenon called "the leans". This might occur when the IFR equipped aircraft has been inadvertently allowed to slowly bank a few degrees and the pilot then makes a quick correction to level the wings. The vestibular system doesn't register the initial bank but does register the wing levelling as an opposite direction bank (away from a wings level attitude) and the pilot's brain produces a leaning sensation while also perceiving from the instrument readings that the aircraft is flying straight and level. The reaction which can persist for quite a while may be for the pilot to lean sideways in her/his seat so that everything feels right!

Pressing on in deteriorating conditions

Most fatal excursions into IMC by ultralight aircraft seem to occur when the pilot freely elects to find a path through or over high terrain beneath an overlaying cloud cover in order to maintain a time schedule but without ensuring that there is a clear way out or back.

Apart from accepting that you will not be able to cope with adverse weather conditions encountered at low levels and thus positively resisting that urge to press on or get home [which urge seems to become quite strong once you have passed the half-way point] the following rules can save your life.

  • Stay in the clear!
    • Watch what is developing around you including behind you.
    • Don't fly towards worsening weather, if you have to change course fly towards better conditions/terrain.
    • In conditions where the METARs indicate little spread between temperature and dewpoint or the air just feels cooler and damp, perhaps a bit drizzly watch out for mist, fog, fractus or scud suddenly forming, particularly in valleys, across ridge-line saddles or on wooded slopes and more so in the late afternoon which, when combined with a compunction to get there before dark, can lead to disaster. Also the gaps in a layer of broken cloud, in front, behind, above or below you, may start to close-in at any time and very rapidly.
    • When any doubt exists make a 180 turn or divert towards better conditions; the accident reports cite too many instances of light aircraft 'controlled flight into terrain' or 'continued VFR flight in adverse weather' because of increasing cloud cover, a lowering cloud base or reducing visibility probably because the pilot thought 'I'll just go a bit further and see what the conditions look like there'.
  • Be wary of lowering cloud and rising ground!
    • If you can't see a gap between the horizon and the overlaying cloud base be absolutely sure you can proceed and be very careful that (a) you are not gradually climbing and losing airspeed and (b) that you don't get into a position where poor visibility precludes making a 180 turn without entering cloud.
  • Be wary of valleys!
    • If you can't see the tops of surrounding hills because of cloud don't fly into a valley unless (a) you can clearly see the exit and the horizon is quite clear and well defined at that end and (b) the valley is clearly wide enough to do a 'U' turn at any time.
    • In addition possible turbulent downflows over the windward slopes warrant some precautions when turning.
  • Be wary of concealed CB and squall lines !
    • When flying below low and mid level unbroken or broken cloud layers cumulonimbus and squall line development may be concealed from view and you may suddenly encounter extreme turbulence, wind shear and very heavy rain with consequent loss of VMC in the worst possible conditions.
  • Don't get caught on top!
    • If caught above what appears to be an extensive cloud layer it is generally wise to turn 180 and climb for a better line-of-sight distance while returning to clearer sky but remember the wind velocity changes with height so what may be a favourable wind at low level may be unfavourable at height.
  • Be wary of sucker holes!
    • If caught out above a cloud layer be extremely wary of descending through a hole in the layer.
    • Such holes tend to suck you in but, if the hole starts to fill or proves not to be wide enough to conduct a safe slow speed descending turn, disorientation may spit you out the bottom.
    • Also you have to be sure that, having descended through the hole, the height of the cloud base, terrain and visibility will allow safe onward passage in VMC under the cloud cover, or at least the option of a safe precautionary landing.
    • Once descending in a sucker hole it may well prove impossible to climb out of it without entering cloud if you change your mind.