Go / no go - how to calculate obstacle clearance

[Loco]

Hi guys

I'm putting together a spreadsheet for myself to assist me with flight planning

Part of the spread-sheet is to make sure I'll be able to take off safely at all runways by having enough clearance over obstacles e.g. power lines

So given the following inputs:
- QNH 1021
- Temp 32 deg. celcius
- Alt 4780 ft
- Humidity 55%
- Aquilla 582
- PIC 110kg
- PAX 65kg
- 50l fuel
- 10kg luggage
- Runway 250m
- Power lines 50m from threshold 20m high

I know I have to:
1. First calc if we're not overloaded by making sure fuel plus load (PIC + PAX + luggage) doesn't exceed 450kg - empty weight (from weight and bal report)
2. Then to calc density altitude and try calc if we'll have enough power+lift to clear the obstacle

But given the density alt how do you calc 'having enough power' to make a go / no go decision with enough safety margin?!

Please can someone help me with this calc

Many thanks

Cheers
Ant

[Loco]

I guess one will have to come up with a formula that will calculate how much take off roll is required to clear obstacle (+ safety margin) which takes above parameters incl. Engine HP and density alt

Then if required take off roll > available runway (+ safety margin) then no go

Surely there must be an existing formula for the above? (My google skills are letting me down ) - how does the PPL's do this and how do you guys decide before taking off eish?!

[Loco]

I see in the Jim Davis book PPL's use the aircraft's POH (Pilot Operating Handbook) to determine if they'll be able to clear obstacles on take-off

This calculation takes into account density altitude calculated purely on Temp & Alt - not QNH!? I would have thought QNH has an impact on DA!?

Anybody seen something similar for Aquilla 582's?

Thanks
Ant

[Relborg]

Found the following on the WITS flying club website:

Hot summer afternoon: temperature is 35 °C and by setting 1013.2 on the altimeter pressure setting scale we read off the pressure altitude as 2400 feet.

The temperature of 35 °C is 24 °C greater than ISA so the density altitude variation due to temperature variation is (plus) 24 × 120 = plus 2880 feet.
So density altitude = 2400 + 2880 = 5280 feet

Thus the aircraft will perform poorly at take-off, probably at less than 70% of its rated sea level capability.

The following is an extract from an RAAus incident report:
"I was attempting to take-off in a paddock approximately 140 metres in length. Due to the hot [35 °C] conditions the aircraft did not get enough lift which resulted in the main wheels catching the top wire of the boundary fence. The aircraft was slowed and struck the ground in a nose-down position. The wire snapped allowing the aircraft to bounce approximately 20 feet in the air. I cut the power and landed the aircraft to the left to miss another fence. This caused the left wingtip to strike the ground before coming to a stop. I walked away from the accident."

The aircraft manufacturer provided the following information: "... the take-off distance to safely clear a 15 metre obstacle is 213 metres in ISA sea level conditions."

Rule of Thumb #1

In the absence of manufacturer supplied data the effect of density altitude on TODR (for a dry, smooth and level surface) can be roughly estimated:-

"In nil wind conditions, for each 1000 feet that the pressure altitude exceeds sea level add 10% to TODR, then for each 10 °C that the airfield temperature exceeds 0 °C add a further 10%."

e.g. the 'Olly's Folly' hot day situation, the aircraft manufacturer's standard sea level TODR is 250 metres.

Pressure altitude is 2400 feet: 250 × 1.24 = 310 metres.

Temperature is 35 °C: 310 × 1.35 = 419 metres TOD.

Then add a further 10% margin for random events = 460 metres estimated TODR, but this is for a dry, smooth and level surface, if the surface is long grass with a 2% upslope then you might have to add another 50% to TODR making it nearly three times the manufacturer's standard distance!

Remember that all the factors mentioned above relating to surface, slope, pressure, temperature, airframe and engine condition are cumulative and the runway length is finite.


Rule of Thumb #2

In the absence of manufacturer supplied data the effect of density altitude on maximum rate of climb at Vy can be roughly estimated:-

Let's say our aircraft's manufacturer states the rate of climb at sea level in standard ISA conditions is 1000 feet per minute at Vy. However manufacturers' standard sea level rates of climb are usually based on an aircraft in factory new condition, flown by a very accurate pilot in the most benign atmospheric conditions. The manufacturer's standard should be downgraded by a factor that represents an adjustment for general engine, propeller, airframe and other conditions – say 15%, thus the practical rate of climb at sea level in standard ISA conditions should be regarded as 850 feet per minute at Vy.

"The practical rate of climb at Vy should be reduced by 10% for each 1000 feet of density altitude."

e.g. At a density altitude of 5000 feet a 50% reduction to 425 ft/min.

[justin.schoeman]

I see in the Jim Davis book PPL's use the aircraft's POH (Pilot Operating Handbook) to determine if they'll be able to clear obstacles on take-off

This calculation takes into account density altitude calculated purely on Temp & Alt - not QNH!? I would have thought QNH has an impact on DA!?

Anybody seen something similar for Aquilla 582's?

Thanks
Ant

I don't think there is a simple formula. The back of the PoH of a certified plane is filled with performance charts, mostly determined experimentally. The ones I have seen only include a few calculations - generally wind and slope compensation factors.

Take-off performance is a very complex calculation. Engine power depends on temperature and altitude. Take off speed (TAS) depends on temperature and altitude (in a different relationship). Angle of climb depends on excess power (which is mostly dependent on engine power, and IAS). Type and quality of the runway surface also plays a large role.

[nicow]

http://wahiduddin.net/calc/calc_hp_dp.htm

[Loco]

Thanks guys for the examples & info

Yeah there's so many factors that come into play:
- aircraft condition (prop etc. if not new)
- engine HP
- runway surface (grass, gravel, tar, sand etc.)
- humidity
- weight
- wind (strength & direction)
- temperature
- runway length
- obstacle height & location
- density altitude
- QNH
- other?

It's frightening that someone hasn't come up with a formula yet where you could punch in these values & it spits out a result that includes a safety margin <--- that tells you either "yes it's safe" or "no you should probably not take off!!!"

I've read some accident & incident reports where this played a role & we're almost left up to take these factors into account & make a judgement call - that's a lot of responsibility on the pilot's part

We need a "propeller head" to come up with the magic formula

Cheers
Ant

[Loco]

http://wahiduddin.net/calc/calc_hp_dp.htm

Hi Nico

Thanks I've had a look at that calculator

How do you personally interpret the results & base decisions from it?

Cheers
Ant

[John.com]

That “black-box” type calculator is far too complicated for the average pilot to understand, let alone USE! Furthermore, it requires a computer!! In the hanger . . . huh? . . . . me thinks not!

What's more, it is never just about "obstacle clearance" . . . . hopefully these notes will explain more!

You need something PRACTICAL that you can use in the pre-flight planning stage when strolling around your aerie and glancing at the wind sock, and which uses your experience and practical knowledge ONLY!

IMHO, here’s my take on the matter . . . . .

Out of everything that gets thrown at you in the pre-takeoff and takeoff phases there are really SIX main considerations to focus on! Get one or more of these wrong and your risk in the takeoff phase increases exponentially!

So here’s what they are:
1. Meteorological Conditions/Weather – including all those criteria such as wind, QNH, humidity, etc.
2. EFATO Risk Mitigation – mitigate EFATO risk through normal maintenance and pre-flight checks focused on ignition and fuel supply systems
3. Weight & Balance Check – ok, so balance is not THAT critical in a trike, but get the weight calculation wrong and it could have really serious consequences
4. Cross-Wind Components - have a higher than anticipated tailwind component pushing you down the runway and you could very well not make it into the air by the end!
5. Runway Distance Check – get this wrong and it has REALLY serious consequences!
6. Obstacle Clearance Check – again, get this wrong and it has REALLY serious consequences!!

Now, before I proceed . . . . MOST of 1 – 6 are done as a matter of course during the standard pre-flight processes and is supported by the many hours of training and experience. So, besides the weather which needs to be studied ON THE DAY, and then interpreted and acted on (fly/don’t fly?), items 2 – 6 are largely done as a matter of course for trike (and other) aviators . . . . with no fancy calculators needed!

WIth that said, it is when any or many of these other factors (2 - 6) become marginal that even the most experienced aviator needs to stop and revert to pen and paper! Knowing WHEN to stop and question the conditions/factors is the key to a safe takeoff.

It remains a sad fact of life that our aircraft (trike) operating manuals contain precious little information relating to the issues raised at the start of this thread. You will need to build this information for your aircraft though copious record keeping after each flight.


OK, so onto the six considerations . . . . .


Meteorological Conditions/Weather

I am not going to pen a sermon on weather here . . . . there are a lot of books and websites out there, suffice to say this: Interpret the weather incorrectly or fly in borderline meteorological conditions and you WILL suffer the consequences!

Here’s ONE example: the only thing worse than taking off in a quartering tailwind is taking off in a quartering GUSTING tailwind!!! Taking off with a tailwind and crosswind component and have it GUST, i.e. possibly quadruple the tailwind component, as you are trying to clear an obstacle on take off and you could be in for a nasty surprise!

Most pilots check the weather before they fly – not doing this is not just poor pilotage, but also against the law! THE AIR LAW DICTATES THAT YOU HAVE TO CHECK THE WEATHER CONDITIONS BEFORE FLYING!

So, yes, in summary, you do need to have a current met report and be able to interpret the effect of this report on not only your takeoff, but your cruise and landing as well.

Enough said . . . .

EFATO Risk Mitigation

We read about it ALL the time and I guess EFATO remains one of every pilot’s worst situations to have to deal with.

There are things you can do to mitigate EFATO risk, recognising that you will never completely eliminate the risk of an engine failure on takeoff and therefore ALWAYS need to be prepared for it.

In short, besides a major engine failure in the form of a crankshaft breaking, etc. EFATO will typically take the form of fuel supply or ignition system failure.

EFATO Risk Mitigation includes:
1. Regular Engine Maintenance in terms of engine manufacturer maintenance guidelines, i.e. for both major and minor servicing
2. THOROUGH Engine Pre-Flight Checks, with a specific focus on the Ignition System (magneto check on engine start and security of spark plug caps and cables) and the Fuel Supply System (dirt/water in fuel, inspect fuel bowls on carbs, cracking in fuel line pipes, security of fuel line hose clamps, overall carburetor security, throttle cables – both hand and foot, vapor-lock, etc.)


So now, the main focus of my writing . . . . the other FOUR major considerations before takeoff:
3. W&B Check
4. Crosswind Component Check
5. Runway Takeoff Distance Check
6. Obstacle Avoidance Check

Again, let me say . . . . this is NOT something I use every time I fly, but I figure that it is damn good to have if, let’s say, I had done a precautionary landing on a farm road and then to take off I had to consider these factors:
> Mid-day – hot & humid
> High Altitude
> High QNH
> Short take off, say 200m, slight downhill
> 10knt quartering tailwind, gusting at 20knts
> Full load of fuel, PAX, tools, etc.
> 5m telephone line to clear at 250m

Do you get where I am going with this? If so, read on . . . . .


I suggest using something similar to the FOUR Practical and Simple Tools (along with one chart) which I use when doing these critical pre-flight checks.

Here is the page out of my Aquilla 582 Operating Check List, followed by the detail on each section.

It takes the form of a 3” x 6” takeoff checklist, comprising a laminated double-sided page which one would use a washable marker pen to write down the information needed/calculated.

Checklist P9 - W&B, Wind and Runway Checks.jpg

OK, so here is the detail . . . .


3. Tool # 1: Weight & Balance Check

The first tool is a W&B test that prompts for ALL the inputs:

Weight & Balance Check.png

So, all quite self explanatory, and ensures ALL that equipment (fire extinguisher, tools, medical, etc.) is included!

Also, remember that it should never be seen to be embarrassing to ask a PAX their weight, albeit a lady . . . . as failure to do so could kill you!


4. Tool # 2 : Crosswind Component Check

The X-Wind Component Tool allows you to quickly and easily calculate the x-wind component and, more importantly, headwind/tailwind component, on the day and not necessarily have to rely on the met report generated in a remote location! Obviously, a tailwind component would have a major negative effect on takeoff distance and obstacle clearance.

Calculating your headwind or tailwind component is about as important as throwing in fuel before taking off!!

REMEMBER: “Gusting is the Killer!” A consistent, albeit stronger crosswind is easier to manage on takeoff than a lighter but gusting crosswind! Remember that gusting happens in “pockets” of air. So, if that “gust pocket” so happens to start or end roughly on the keel-line of your trike it could put your one wing into an instant and dramatic stall . . . . . and at 10m off the deck the outcome will speak for itself! Alternatively, if the tailwind component from a gust hits you square on from the rear you as you are climbing out you can expect your climb rate to diminish dramatically, if not completely!!

So, take time to check out the windsock, carefully gauge the wind speed and LOOK FOR GUSTING!!!

Then, use this useful tool to calculate headwind or tailwind components as they BOTH effect your takeoff roll distance dramatically!

X-Wind Check.png

5. Tool # 3 : Runway Takeoff Distance Check

Then, the third tool is a Runway Takeoff Distance Check, with workings largely based on record-keeping from previous "safe" flights where conditions were favourabe, takeoff criteria (QNH, humidity, etc.) were within safe norms and overall weight was less that 75% MTOW!

Runway Distance Takeoff Check.png

So, to expand a little:

1. At the airstrip where you fly from you would need to measure by pacing off points to know exactly where you are getting airborne - it is important to record this, as YOU need to develop YOUR set of criteria for taking off in YOUR aircraft, all factors considered, i.e. condition of aircraft, engine HP, runway surface (grass, gravel, tar, sand etc.), humidity, takeoff weight, wind (strength & direction, so headwind/tailwind component), temperature, runway length, obstacle height & location, density altitude, QNH, etc.

A quick question: How many trike pilots can honestly say that they know (to the meter) their take off distances (solo, dual, full fuel load, etc.)?

2. Then, on a LONG runway and in zero-wind conditions, calculate stopping distance by aborting your takeoff JUST as your aircraft is about to lift off and then noting the distance it took you to stop! Trikes, especially those with drum brakes on the front wheel only, are notorious for fairly extended stopping distances! SO, if for example you then found yourself taking off on a DOWNHILL runway, you would need to know to add 25% (or more, depending on the decline) to your normal stopping distance in the event of aborting the takeoff!! THIS IS KEY IN TERMS OF DETERMINING YOUR ABORT POINT/POINT OF NO RETURN!!!

3. If you feel your trike's takeoff criteria are borderline, you can then use Pressure Altitude and Airport Temperature and refer to the Koch Chart (see below) to determine what to factor your takeoff length by and thereby determine the INCREASED TAKEOFF LENGTH you would need! The nice thing about this is that it is a little chart that you keep on you, so no fancy computerised online black-box needed!

4. Now, one of the BEST rules to obey is THIS: the 50/70 Rule: you must be at 70% Takeoff Airspeed at 50% of the Runway!!! Again, you need to know your USABLE runway length! If at a runway you don't know and take-off criteria are borderline, and say you have an obstacle to clear, you would need to pace off the runway and determine the halfway (abort) point if 70% of Vto (i.e. 32mph of an Aquilla where Vto = 45mph) is not achieved - THEN, you must STILL have enough distance to STOP!!!! i.e. within the second half of the runway!!

5. Then, based on YOUR criteria you have built up when taking off well within your aircrafts performance envelopes, you would THEN be able to SAFELY determine "Fly/Don't Fly?" with respect to runway lengths by using the simple format above!

Finally, one of the easiest ways of determining these runway values is by using Google Earth and then by using the Ruler Tool to measure dimensions. As an example, here is Aeroden’s runway 36, with dimensions taken directly from Google Earth.

Aeroden Layout.jpg

A : Solo Take Off – Full Fuel – 140m (for ZU-BST - Aquilla 582)
B : Dual Take Off – Full Fuel – 160m (for ZU-BST - Aquilla 582)
C : 50% of USABLE Runway – 350m
D : Total Runway – 850m
E : Usable Runway – 700m (cables at end of 36, 10m clearance needed on cables at 5m, so 150m "run" & 15m "rise" gives a 1:10 climb rate, which in turn, looking at the table on the tool below, indicates a required climb rate of 450 ft/m @ 50mph - something VERY achievable!)


6. Tool # 4 : Obstacle Clearance Check

This is the toughest of them all as EVERYTHING comes into play! Runway length, wind speed, gusting and direction, weight, climb rate, etc., etc.

The Golden Rule is this: If you have an obstacle to clear on takeoff and ANY AIRCRAFT, WEATHER OR AIRFIELD CRITERIA IS BORDERLINE – DON’T START YOUR ENGINE!!!!

OK, so again, your data needed for this tool is largely obtained through “hard miles”, i.e. experience and record keeping over many takeoffs in varied conditions.

Obstacle Clearance Check.png

The steps are:
1. Measure the distance between planned take-off and the the obstacle to clear, in meters (X)
2. Estimate the height of the obstacle to clear, in meters (Y)
3. Decide on a safety margin for the obstacle to clear (not less than 10m)
4. Calculate the Climb Ratio by dividing X by (Y + 10m), so X / (Y+10m)
5. Use the Ratio and the table provided to VERIFY that you can achieve the required Climb Rate in ft/m

To calculate takeoff gradients/ratios/angles for your own aircraft (varied ground speed and rate of climb) you can use this Slope Calculator (http://www.1728.org/gradient.htm).

IMPORTANT NOTE: This Obstacle Clearance Check tool outlined above is for ZERO headwind/tailwind conditions! Clearly a headwind would assist in the process of clearing an obstacle but a tailwind introduces a number of variables into the calculation that would require a more scientific approach. Unless you know the airstrip/runway VERY well and have experience in tailwind component takeoffs DO NOT venture into this realm!


Chart # 1 : Koch Chart

Koch Chart.png

DISCLAIMER: The onus rests with the PIC to double check these tools and calculations before using them for flying.

These four Takeoff Check Tools are ALL contained on a single back-to-back laminated page that measures 75mm x 150mm . . . . so small enough to carry with you anywhere! All that you then need is a washable marker pen for writing on the laminated surfaces!

Unfortunately the pdf of the tool is over 100KB so if any of you would like a copy of the Takeoff Check Tool you can PM me with your email, aircraft registration and Vto (takeoff speed) and I will forward you a "personalised" copy to you. Then just print, cut-out and laminate back-to-back.

I hope that this helps some of you out there!

Any comments/suggestions for improvements would be most welcome!


Safe Skies!

John

[Loco]

Amazing post thank you Sir!!!

[John.com]

Found the following on the WITS flying club website:

Hot summer afternoon: temperature is 35 °C and by setting 1013.2 on the altimeter pressure setting scale we read off the pressure altitude as 2400 feet.

The temperature of 35 °C is 24 °C greater than ISA so the density altitude variation due to temperature variation is (plus) 24 × 120 = plus 2880 feet.
So density altitude = 2400 + 2880 = 5280 feet

Thus the aircraft will perform poorly at take-off, probably at less than 70% of its rated sea level capability.

The following is an extract from an RAAus incident report:
"I was attempting to take-off in a paddock approximately 140 metres in length. Due to the hot [35 °C] conditions the aircraft did not get enough lift which resulted in the main wheels catching the top wire of the boundary fence. The aircraft was slowed and struck the ground in a nose-down position. The wire snapped allowing the aircraft to bounce approximately 20 feet in the air. I cut the power and landed the aircraft to the left to miss another fence. This caused the left wingtip to strike the ground before coming to a stop. I walked away from the accident."

The aircraft manufacturer provided the following information: "... the take-off distance to safely clear a 15 metre obstacle is 213 metres in ISA sea level conditions."

Rule of Thumb #1

In the absence of manufacturer supplied data the effect of density altitude on TODR (for a dry, smooth and level surface) can be roughly estimated:-

"In nil wind conditions, for each 1000 feet that the pressure altitude exceeds sea level add 10% to TODR, then for each 10 °C that the airfield temperature exceeds 0 °C add a further 10%."

e.g. the 'Olly's Folly' hot day situation, the aircraft manufacturer's standard sea level TODR is 250 metres.

Pressure altitude is 2400 feet: 250 × 1.24 = 310 metres.

Temperature is 35 °C: 310 × 1.35 = 419 metres TOD.

Then add a further 10% margin for random events = 460 metres estimated TODR, but this is for a dry, smooth and level surface, if the surface is long grass with a 2% upslope then you might have to add another 50% to TODR making it nearly three times the manufacturer's standard distance!

Remember that all the factors mentioned above relating to surface, slope, pressure, temperature, airframe and engine condition are cumulative and the runway length is finite.


Rule of Thumb #2

In the absence of manufacturer supplied data the effect of density altitude on maximum rate of climb at Vy can be roughly estimated:-

Let's say our aircraft's manufacturer states the rate of climb at sea level in standard ISA conditions is 1000 feet per minute at Vy. However manufacturers' standard sea level rates of climb are usually based on an aircraft in factory new condition, flown by a very accurate pilot in the most benign atmospheric conditions. The manufacturer's standard should be downgraded by a factor that represents an adjustment for general engine, propeller, airframe and other conditions – say 15%, thus the practical rate of climb at sea level in standard ISA conditions should be regarded as 850 feet per minute at Vy.

"The practical rate of climb at Vy should be reduced by 10% for each 1000 feet of density altitude."

e.g. At a density altitude of 5000 feet a 50% reduction to 425 ft/min.

Good information Dirk!

Do you have a reference for the "rules of thumb"?

This is something that we could probably use generically for trikes and not be far wrong, especially when you consider the additional 10% for margin!

Many thanks for sharing this.

Safe Skies!

John

[John.com]

Oh, ok . . . I see Wits Flying Club! I will check for their references!!

Thanks again!

[John.com]

Amazing post thank you Sir!!!

Glad I could help Anthony!

I have just read Dirk's post again and re-digested the two "Rules of Thumb"! Very good info.

I have sent this text to my mobile phone, which means that I will have it on hand if ever conditions/criteria are marginal and I need to revert to pen and paper!

It is so important to integrate what the two of us have posted here. One (Dirk's post), the theory of getting to a TODR on the day, for a given AD, pressure altitude and temperature, and the other (my post), the practical way of checking that TODR is OK for the airfield in question.

The only real concern is that the POH for an Aquilla does not provide any information on TODR at sea level at zero degrees C!!

Aquilla POH - Takeoff.png

Don't you just love "When all obstacles have been cleared, reduce the throttle setting as required"?

I would hazard a guess at TODR for an Aquilla, PIC only, full fuel load, being 125m at sea level at 25°C, nothing less!

Safe Skies!

John

[Relborg]

Oh, ok . . . I see Wits Flying Club! I will check for their references!!

Thanks again!

[Tumbleweed]

OK.This is my opinion (after some quality hangar talk) and not an instructor's.

You gonna squabble your brain with overly complex stats and pre flying specs. Land at a strange field after an hour's flip to find a crosswind take off on long grass with a slight gradient and you'll ponder with your calculations and you're going to be a nervous wreck.

Establish your take off distance fully loaded, single and with pax in;
(a) sterile conditions -slight head wind (b) cross wind (c) tailwind. Try and establish if your height at 400 -500 feet from take-off is comfortable enough to clear trees/ wires e.t.c.

You'll probably find that that you need 10 % more runway to clear the same height in a cross wind and 20% more with a tailwind take off.

If you 10h00 take off is grass. maybe wet, add another 10 to 20% on your distance.

You might have burned off 20 litres from your first take off but densisty altitude might tax you 20 % performance loss which you need to add to your minimum take off distance.

So, if your sterile take off distance is 150 metres (including tree clearance of 20 feet), add 10% for pax, 20 % for tailwind, 20% for density altitude and 10% for a crappy runway then you should'nt have landed at a strip shorter than 250 - 300 metres.

Either loose the pax or wait for the head wind take-off.