I think every table I have played at where it has come up characters are able to choose to fail a save if they wish but I can not find anywhere it is included in the rules.

There are some spells like polymorph that are frequently used on both enemies and allies and spell like this refer to the save only needing ot be made by an unwilling creature but want about spells that do not specify this.

in my campaign my druid and an NPC were forced off an airship in combat, someone cast feather fall on usand then the NPC cast levitate and started moving me up with a request ot pick him up after combat. On my turn I asked the DM how high we were, after a successfull check I was told I know the captain usually cruises at 500ft and that matches an estimate from what I can see of the ground. The reason I asked was that if we were more than 600 ft up  I would cast earthbind on the NPC so he would not plunged to his death after a minute. Earthbind says "The target must succeed on a Strength saving throw, or....." so succeed on the saving throw and he dies, fail the saving throw and he lives, which seems the wrong way round to me. RAW is he allowed to choose to fail the save?

Technically, unless the text says you can or similar, then by RAW no. However, I think many agree that it is a dumb rule so choosing to fail is a common houserule.

No you can't willingly fail a saving throw you are forced to make it. 

Saving Throw:  A saving throw--also called a save--represents an attempt to resist a spell, a trap, a poison, a disease, or a similar threat. You don’t normally decide to make a saving throw; you are forced to make one because your character or monster is at risk of harm.

Quote from Plaguescarred >>

No you can't willingly fail a saving throw you are forced to make it. 

Saving Throw:  A saving throw--also called a save--represents an attempt to resist a spell, a trap, a poison, a disease, or a similar threat. You don’t normally decide to make a saving throw; you are forced to make one because your character or monster is at risk of harm.

In an attempt to resist the effects. What if you do not choose to resist?

Ultimately, like any ruling at a table, up to the DM as to whether or not you may be permitted to willingly fail a saving throw to allow a spell effect to go through

Quote from Erik_Soong >>

 

In an attempt to resist the effects. What if you do not choose to resist?

 A saving throw represent an attempt to resist an effect and therefore it must be attempted whenever one is called for, unless noted otherwise, such as Calm Emotions, which specifically let you choose to fail the save.

Quote from Erik_Soong >>

Quote from Plaguescarred >>

No you can't willingly fail a saving throw you are forced to make it. 

Saving Throw:  A saving throw--also called a save--represents an attempt to resist a spell, a trap, a poison, a disease, or a similar threat. You don’t normally decide to make a saving throw; you are forced to make one because your character or monster is at risk of harm.

In an attempt to resist the effects. What if you do not choose to resist?

 
RAW that resistance is automatic or instinctive. Only effects that specifically state “a creature can choose to fail this save” (or something similar) grant a creature the option to not resist the thing.

A save represents something resembling a reflex in my opinion. RAW, as many others have stated, there is no choice unless the feature forcing the save says so in its description.

I think that’s mechanically the distinctive difference between a save and a check, though I would point out that initiative seems to be firmly in the middle somewhere.

the element of choice means that to some degree there’s enough time to contemplate and make the choice.

a check is something that mostly seems to be intentioned and known about before hand.

removing your hand quickly from a hit object you’ve touched to prevent further pain would be a save.

preparing to to hold an object for a long time that you know is hot and causes pain seems like it would be a check.

being forced to continually and repeatedly make saving throws for an effect seems dubious to me, but it is a common way to balance features over time. Features that allow the effected creature to spend an action to make a saving throw are even more confusing to me.

Quote from Plaguescarred >>

Quote from Erik_Soong >>

 

In an attempt to resist the effects. What if you do not choose to resist?

 A saving throw represent an attempt to resist an effect and therefore it must be attempted whenever one is called for, unless noted otherwise, such as Calm Emotions, which specifically let you choose to fail the save.

Quote from IamSposta >>

Quote from Erik_Soong >>

Quote from Plaguescarred >>

No you can't willingly fail a saving throw you are forced to make it. 

Saving Throw:  A saving throw--also called a save--represents an attempt to resist a spell, a trap, a poison, a disease, or a similar threat. You don’t normally decide to make a saving throw; you are forced to make one because your character or monster is at risk of harm.

In an attempt to resist the effects. What if you do not choose to resist?

 
RAW that resistance is automatic or instinctive. Only effects that specifically state “a creature can choose to fail this save” (or something similar) grant a creature the option to not resist the thing.

Thank you both!

Disagree with what's being stated as RAW.   It's still unclear due to wording of the what is written.
I'll break down my reasoning:
Saving Throw:  A saving throw--also called a save--represents an attempt to resist a spell, a trap, a poison, a disease, or a similar threat. You don’t normally decide to make a saving throw; you are forced to make one because your character or monster is at risk of harm.

1) "at attempt to resist" - this indicates you are attempting to resist.   Much like you attempt to avoid being hit hit with AC - normally - but could choose to simply take the hit.  As stated by others - there are specific spells which offer the choice to fail the save, which argues for the intent of how to read this(and I'd agree with that - in general play - no... not always, DM side - maybe I want the BBEG to just take a specific spells full damage - I COULD DM fiat that - but - if it's standard practice to allow Players to fail a save when then want to also - fair and impartial has been be maintained), but no - RAW is not clear.

Will players figure out some way to use this?  derp - of course.
Does it benefit specific specific encounters / monsters to fail specific saves?  also - of course.  Why exactly would a Flesh Golem save vs. any lightning effect? It's doesn't fall under "attempt resist a (Theat)" - it's literally attempting to resist a heal in this case.

2) "You don't (and this is key) *normally* decided to make to a saving throw; you are forced to make one because ..."(etc)  Normally - RAW is not a magic spell that somehow make poor wording and unclear intent crystal clear.  Language is vital in game design - and D&D 5e suffers for lack of clearly defined keywords, so you at least know when you're off script or out in the territory of house rules.  The same passage can be used to justify not allowing a saving throw at all (literally because of how it wording), which I think most (including myself) would argue is not the intent.  RAW - clear as mud, easily manipulated to whatever you want it to mean in this case.  Commonly held belief of the intent is what's really being discussed here.

another key point - IS AT RISK OF HARM - yes - you can choose to fail any save at anytime that does not involve risk of harm.  Which can easily be used to justify failing saves all over the place (this is IMO likely the reason why some specific spells point out the options to fail save, they lack a risk of harm on a failed save).

3) conclusion - you decide what you want it to mean with your group (don't be an at MY table person - there's for a word for that too - tyrant, be that at OUR table group).

Like the Devs say '@JeremyECrawford  No rule lets you opt to fail a save.... As DM, I might allow it, assuming you aren't incapacitated or dominated.''  so when any effect say you must make savint throw, you have to unless the DM lets you automatically  suceed or fail it.  

It doesn't seem that there is anything left to debate here, the responses have been very clear and thorough. I think it good to have a clear understanding of the rules so you can choose which ones to use or ignore from an informed position. Just like the rules for Uno say you cannot play a +2 on a +2, I now understand this rule fully, and will immediately disregard it. :)

A real life example in the game I DM for is that a Zone of Truth spell was needed to convince an NPC of something vital to the story. The face had failed the check. The players recommended that the NPC cast the spell and volunteered to fail the save so they could address concerns that the NPC had. At the time, I had allowed them to avoid the save and just choose to submit to the effects of the spell. I am happy to have learned that this was not in accordance of the rules thanks to the tireless and very much appreciated efforts of Plaguescarred and Iamsposta, but if I could go back and do that scene over, I would change nothing.

You actually do have the explicit option to fail the save in Zone of Truth, but that just makes it a specifically designated exception to the rule.

Edit: My bad, on review turns out you don't get the option for this one. However, given its duration and the fact it is essentially forcing the save every 6 seconds, it's pretty reasonable to handwave the fail.

Quote from Erik_Soong >>

I am happy to have learned that this was not in accordance of the rules thanks to the tireless and very much appreciated efforts of Plaguescarred and Iamsposta, but if I could go back and do that scene over, I would change nothing.

 You're welcome i'm glad you appreciate! No change needed , your DM ruling is possible by applying a situational penalty so important that the saving throw will fail, ex. - 33 ;)

Saving Throw: A saving throw can be modified by a situational bonus or penalty and can be affected by advantage and disadvantage, as determined by the DM.

It appears the 2024 rules have made it explicit: Saving Throw

A saving throw—also called a save—represents an attempt to avoid or resist a threat. You normally make a saving throw only when a rule requires you to do so, but you can decide to fail the save without rolling. The result of a save is detailed in the effect that allowed it. If a target is forced to make a save and lacks the ability score used by it, the target automatically fails. See also chapter 1 (“d20 Tests”).

Quote from Dwayden >>

It appears the 2024 rules have made it explicit: Saving Throw

A saving throw—also called a save—represents an attempt to avoid or resist a threat. You normally make a saving throw only when a rule requires you to do so, but you can decide to fail the save without rolling. The result of a save is detailed in the effect that allowed it. If a target is forced to make a save and lacks the ability score used by it, the target automatically fails. See also chapter 1 (“d20 Tests”).

 

 Yeah, one of the subtle changes in the 2024 PHB.